Nowadays, from the perspective of dental preservation, the pulpal and periapical lesions caused by caries or non-carious diseases are removed or controlled by various means, so that the apical lesions can be restored and the preserved teeth can perform normal masticatory functions. Throughout the 200 years of history of treatment of periapical disease, there have been several stages, and there is still no strict standard for the evaluation of root filling and healing criteria today. The first type of healing, where the affected tooth is able to perform normal masticatory functions, is the gold standard. In other cases, the periodontal space is widened to scar connective tissue. Although this is also a form of healing of periapical disease, the tooth is sore, slightly painful and uncomfortable when chewing hard food for a long period of time, while chewing normal food is not uncomfortable, or the condition may be lighter or heavier, which is still strictly speaking an unsuccessful case. Only after root canal preparation, disinfection, and root filling to isolate the periapical and pulpal cavities, the intra-apical infection cannot continue to invade the periapical tissue and bone. The periapical inflammatory tissue was removed by phagocytes, and the granulation tissue was transformed into connective tissue, differentiated into dental osteoblasts and osteoclasts, deposited dental bone at the apical foramen, and finally closed the apical foramen. x-ray showed that the apical sparse area disappeared, the new sclerotic plate image was clear, and the periapical space was restored to its original width. At this time, the affected tooth is stable without loosening and can perform normal chewing function as ideal healing. In order to achieve ideal healing many experts and scholars have explored many treatment methods, such as live pulpotomy, pulp capping, dry pulpotomy, pulp extraction, plastic surgery, and root canal treatment, although each treatment has its own clear indications. However, in the case of chronic periapical infection, I am afraid that endodontics is the only reliable option. Root filling, that is, the narrowest part of the apical foramen is located not at the opening but at 0.5-1 mm from the opening, is particularly important. The apical foramen is required to be kept as small as possible in modern endodontic surgery. The aim is to keep the apical foramen as small as possible after root preparation. After the removal of various pathological factors, its own defense repair can deposit dental bone and eventually close the apical foramen faster and restore the chewing function sooner. The narrowest position is kept unchanged, the apical bone is less damaged, and after the pathogenic factors are removed, the new bone formed by the differentiation of connective tissue into osteoblasts can be connected with the original bone early and the apical foramen is closed. For the formation of dentin bridges and closure of the apical foramen as mentioned in the textbooks in the past, we cannot agree because after root preparation, regardless of whether there is living pulp in the original apical part, there is no dentin-making cells after root preparation, so it is impossible to generate new dentin and close the apical foramen. If the pulp is necrotic and periapical tissue resorption and destruction, there is no way to talk about the generation of dentin and the closure of the complete apical foramen. The normal restoration process should be: complete elimination of pathogenic factors – the original inflammatory tissue mechanization, resorption – connective tissue differentiation out of adult osteoblasts (from the living periodontal membrane) and osteoblasts (from the alveolar bone) – healing of periapical lesions. The healing is mainly through regeneration and repair of periodontal tissues. Therefore, the only ultimate goal of our root preparation, disinfection and root filling process in the treatment of periapical disease is to close the root canal tightly to prevent the infection of the root by pathogenic factors in the root and dentin tubules, and to create conditions and environment for the healing of periapical lesions. There are many methods of root preparation, disinfection, and root filling, and whatever method is used, it is possible as long as the conditions permit and the operation is convenient. The only important thing is the correct treatment of the apical part. For cases where the apical foramen is not damaged and changed, normal healing is possible with proper filling or with only a little paste overfill. We have filled a case of underfilling of the adhesive tip and overfilling of the paste with little apical lesion more than ten years ago, and the affected tooth has been preserved and chewed normally. This shows that in the case of minor apical lesions, there can be a phase of healing without proper filling. In cases where the apical foramen has been destroyed without treatment or due to improper handling by the surgeon. If it is possible to restore the apical foramen, the apical foramen can be restored with the aid of root canal endoscopy or microscopy using calcium hydroxide, MTA, etc. to preserve more of the affected tooth. In addition, we recommend that regardless of the root canal disinfection, root filling agents, and various pharmaceutical materials used for apical foramen restoration, minimize their irritation, such as phenols, which are extremely detrimental to the healing of periapical lesions due to their protoplasmic toxic effects.