The oral cavity is an integral part of the organism. Certain systemic diseases can show some features in oral tissues, especially in oral mucosa and periodontal tissues, and these features can be used as an important diagnostic basis for systemic diseases. 1, blood system diseases, including leukemia, hemophilia, granulocytopenia, thrombocytopenic purpura, anemia, etc., such as leukemia in the oral cavity, the significant manifestations of the gum hyperplasia hypertrophy, pale, necrotic or local bruising dark red, gums often have spontaneous or multiple bleeding, light and sticky color, not easy to stop bleeding. Other blood disorders may also appear as gingival bleeding, gum enlargement, oral mucosal ulceration or necrosis, toothache or loose teeth. When performing oral treatment, attention should be paid to the taking of a complete medical history and examination of the systemic condition. Treatment should not be limited to local and ignore the systemic diseases, so as not to cause adverse consequences. 2, endocrine disorders and metabolic disorders, such as women in adolescence and menstruation, pregnancy, gingivitis, oral ulcers, gingival congestion, swelling, pain, bleeding. Pregnant women are also prone to gingival tumors in pregnancy. Menopause is prone to burning pain in the mouth and abnormal taste sensation. Hyperthyroidism (hyperthyroidism) can affect the development of the jaws and teeth, causing premature tooth eruption in children as well as contributing to periodontal tissue disease. Hypothyroidism can lead to malocclusion, chronic periodontitis and tooth decay. Diabetic patients with poor oral hygiene can develop a variety of oral manifestations, such as gum congestion, edema, easy bleeding, easy to occur periodontal abscesses, and can also form a large amount of tartar in a short period of time, resulting in periodontal atrophy, tooth neck exposure, tooth hypersensitivity, etc. In addition, they can also develop dry mouth, dryness, and tooth decay. In addition, dry mouth, thirst and rotten apple taste in the mouth can also occur. 3. Immune diseases, such as lupus erythematosus, can be manifested as large vesicles, erosion and necrosis of oral mucosa, and oral discoid erythema and facial lesions can also appear. Other diseases such as scleroderma, dry syndrome, white stuffing syndrome, etc. can have different oral manifestations, and drug allergy can also manifest as oral mucosa damage. 4, nutritional deficiency diseases, such as vitamin C deficiency caused by scurvy (vitamin C deficiency disease) can lead to acute gingivitis, accompanied by bleeding and ulcers. Vitamin B2 deficiency can lead to xerostomia. Other deficiencies such as vitamin B1, folic acid, niacin, vitamin A, protein, iron, etc. can appear oral manifestations. 5. Certain infectious diseases, such as measles, can cause inflammation of the mouth, with congestion and small raised rashes on the oral mucosa. Scarlet fever can appear early with inflammation of the pharynx and cheeks and oral cavity, often extending to the tongue and palate arch, soft palate, and uvula, and petechiae can appear. Mumps, on the other hand, affects the parotid glands, causing congestion of their ductal openings. Tuberculosis, leprosy, syphilis, etc. can also appear oral damage. 6, mercury poisoning, lead poisoning, arsenic poisoning, bismuth poisoning, phosphorus poisoning, etc., can cause damage to the oral mucosa and gums, such as gingivitis, periodontitis, ulcers and a special metallic smell in the mouth and other manifestations. Various kinds of poisoning also have certain special manifestations, for example, chronic lead poisoning can appear lead lines, i.e., gray-blue lines about 1mm wide with blurred edges on the margins of the gums, lips, cheeks and tongue, about 1mm from the inside of the free gingival margin. All of the above systemic diseases, because of the obvious oral manifestations, should not neglect the local treatment of the oral cavity if the diagnosis is clear and when the systemic treatment is the main focus.