Often nail manifestations are merely informative and non-specific for certain diseases. However, when a linear black nail appears, it may be a precursor of melanoma. Linear black nails are mainly caused by the gradual development of onychomatous nevus, which can change from small to large with age, which may lead to the development of melanoma. When a melanin-containing mole begins to appear at the root of the nail, it can gradually extend from the root to the outside tip of the nail, resulting in a black line appearing inside the nail, which is manifested as a linear black nail. When there is a black line on the nail, it is usually mistakenly thought to be due to calcium deficiency, heredity, and other reasons. Thus, when this happens, one should be immediately alert and should go to the dermatology department of the hospital as soon as possible to rule out malignant diseases as the cause, so as to avoid delaying the condition and causing incalculable consequences.