There are many causes of blackened gums and they cannot be generalized. For different cases, corresponding measures must be taken to prevent or deal with them. 1) Poor fit of the gingival margin of the crown, i.e. insufficient precision between the crown and the polished and prepared tooth. The gap between the two should not be larger than 0.1mm, and too large a gap, too thick or too thin crown edge will stimulate the gums and lead to gingival inflammation. In chronic inflammation, the gums usually appear purplish and darker than normal gums. At the same time, the excessive gap will be occupied by the adhesive after bonding, which tends to cause pigmentation and appears black through the gums. In this case, the only way to solve the problem is to remove and redo it. When redoing, gingival row treatment, fine impressions and fine fabrication are very crucial. 2) Secondary caries of teeth at the gingival margin. The primary cause of secondary caries is still poor marginal fit, gap, bacterial accumulation to produce acid to cause caries and gingival blackening. In addition, poor bonding causes bacteria to enter the gap between the crown and the tooth and cause caries. The treatment of this situation must be removed and redone as soon as possible to avoid lesion development. 3) The opaque characteristic of the crown causes the gum to be gray. Normal teeth are translucent, and when light hits the teeth, the teeth are illuminated while they themselves can become a secondary luminous body that can illuminate the gums as well. The porcelain crown layer is opaque metal, the crown is worn, similar to a polished tooth wearing a black mask, the tooth can not form a secondary luminous body, the gums at the edge of the crown lack of internal illumination and obscure. This situation is due to the characteristics of the metal material inside the porcelain crown, the lack of effective means to avoid. It can be improved by using porcelain dental techniques with all-porcelain shoulders. To obtain the desired aesthetic effect, only crown restorations made of all-porcelain materials with optical properties close to those of natural teeth can be used, but they are more expensive. 4) The toxic effect of ions released in the metal used to make the crown. The release of nickel and beryllium ions in nickel-chromium alloy corrosion leads to the denaturation of fibrous tissue in the local gums, followed by discoloration due to endogenous pigmentation in vivo. In this case, even if the old porcelain crown is removed and recreated, the darkened gums are difficult to recover. However, clinical observation shows that the darkening phenomenon will dissipate after a long time. 5) Darkening of the gums due to color change of the abutment itself after polishing. When the pulp of a tooth is necrotic, the sulfide produced acts with protein and iron in body fluids to produce substances such as iron sulfide, which can stain the entire tooth, including the root, blackish brown, and the blackened color of the tooth neck and root penetrates through the gums and is expressed as blackness of the gums. There is no good way to solve this cause of gum blackening. You can try to alleviate it by bleaching inside the root of the tooth before restoration. On the other hand, the importance of preserving the pulp when making crowns is also illustrated. In the case of 1), 2) and 4) above, the crown must be removed and redone due to the problem of its own fabrication or material. For the other cases where marginal accuracy is not a problem, they should be handled depending on the patient’s own aesthetic requirements and clinical specifics.