A brown vertical line inside the nail is considered to be related to trauma, onychomatous nevus, subnail melanoma, etc. According to the cause of the disease, temporary observation or surgical excision should be taken. 1. Trauma: when the nail is smashed or squeezed accidentally, it can cause bruising under the nail, forming a brown vertical line. If there is no other symptom, it can be observed temporarily, and the bruising under the nail usually disappears gradually with the passage of time. 2. Nail nevus: it is a benign skin tumor growing in the area of nail, and there is no effective medicine to take it out, usually there is no uncomfortable symptom, it can be observed temporarily, if it affects the aesthetics or malignant changes, it can be surgically excised. 3. Subnail melanoma: it belongs to one of the types of malignant melanoma, and should cooperate with doctor for surgical intervention as soon as possible to avoid further development. There may be other reasons for the presence of a brown vertical line in the nail. Patients are advised to go to the hospital to determine the cause and follow the doctor’s instructions for treatment and therapy.