1.Skin tumor tissue sources: most common are epidermal, vascular and skin appendage sources, other sources are fibrous tissue, adipose tissue, lymphatic tissue, etc. Rarely, there are special cellular malignancies such as melanocytes and Langerhans’ cells. 2. Common malignant tumors of the skin and their prevention and treatment: namely, basal cell carcinoma (BCC), squamous cell carcinoma (SCC) and malignant melanoma. The important risk factors are ultraviolet radiation and population aging, and the means to confirm the diagnosis is surgical excision for histopathological examination. The prevention of SCC is based on the prevention of sun exposure (avoiding sunlight from 10:00 am to 4:00 pm), avoidance of carcinogenic factors (such as skin radiation, consumption of arsenic-containing compounds), and early diagnosis and treatment of precancerous lesions (such as mucocutaneous leukoplakia, solar keratosis, proliferative erythema, Bowen-like papulosis, and chronic ulcers) from childhood onwards. Treatment is preferred to complete surgical excision, followed by radiotherapy or chemotherapy. Photodynamic therapy is also available for large superficial BCC and precancerous lesions, as well as for those who are too old to undergo surgery. 3.Early diagnosis of malignant melanoma: pay attention to ABCDE changes, i.e. pigmented nevus or dark spot appears A – asymmetric shape, B – irregular boundary, C – uneven color or miscellaneous color, D – lesion diameter greater than 1cm, E – lesion elevation or fast increase, especially with bleeding or itching discomfort. Any single mole or pigmented spot found to have one of these five changes should be excised for pathological examination. It should also be noted that non-pigmented malignant nevus is easily misdiagnosed clinically as hemangioma or septic granuloma. 4, malignant black susceptibility factors: there are genetic factors (skin type such as type I or II skin that is easy to sunburn not easy to tan, photosensitivity, certain genetic diseases such as pigmented dry skin disease), environmental factors (sun exposure, trauma or stimulation, viral infection), autoimmune changes (mutation, immune response). Therefore, people with a family history of malignant tanning or genetic diseases (albinism, pigmented dry skin disease, etc.), a history of sun allergy, and long-term outdoor or field activities are at high risk. Those with congenital nevus and adult-acquired nevus have greater malignancy than those with nevus in childhood. 5. Paraneoplastic dermatoses: i.e., the dermatoses have a high incidence of concomitant tumors. The main ones are paraneoplastic aspergillosis, paraneoplastic acanthosis, malignant acanthosis nigricans, necrolytic loosening erythema, creeping erythema, acquired fetal hair hyperplasia, and Leser-Trelat sign (sudden onset of many seborrheic keratoses). When the above skin manifestations are found, attention should be paid to search for visceral tumors and CT scan should be performed if necessary.