If there is a black vertical line on the thumb, you should first judge whether it is a stain or not. If you exclude stains, the usual causes are trauma, nail fungal disease, onychomycosis, malnutrition and so on. 1. Trauma: If the thumb has been traumatized, resulting in bleeding and bruising, causing excessive local melanin accumulation and poor blood circulation, then black vertical lines can appear locally, usually accompanied by obvious pain, if the wound is not particularly serious, it will usually be absorbed by itself. 2. Nail fungus disease: usually due to fungal infections, can be manifested as abnormal nail color, resulting in a black vertical line on the thumb nail, can also cause nail deformation, cracks, brittle and so on. 3. Onychomycosis: It is a type of nevus, which usually manifests as a black line growing under the nail, and is usually not accompanied by symptoms such as swelling or pain. If the color of the line under the nail is darker and thicker, it is recommended to go to the hospital for dermatopathological examination in time to determine whether the naevus is benign or malignant. 4. Malnutrition: As the growth of nails needs nutritional supply, when the body is malnourished, especially the lack of vitamin B, it may cause the symptoms of unevenness and long black lines on the nail area, so it will also lead to a black vertical line on the thumb. When there is a black vertical line on the thumb, it is recommended to consult a doctor in time for a clear examination and targeted treatment.