The possible causes of fine black lines on finger nails include onychomycosis, nail trauma, onychomelanoma, nail biting fetish, etc., which can be improved by medication and surgical treatment. 1. Nail fungus: many kinds of fungi infringe on the nail area can lead to the occurrence of nail fungus, mainly manifested as cloudy nails, thickening, separation and other symptoms, a few patients can also appear nail fine black line symptoms. Can be applied locally amorolfine nail coating, cyclopia and other drugs to help treatment, serious cases can be taken orally at the same time terbinafine, itraconazole and other antifungal drugs. 2. Nail trauma: when the nails are extruded, impact and other trauma will lead to the nail bed capillary rupture, resulting in bruising under the nail, appearing on the fingernail with a small black line of symptoms. Generally there is no need for special medication, with the growth of the nail bruises can slowly subside, but be careful to avoid re-squeezing, injury to the nail. 3. Nail melanoma: some patients with tiny black lines on finger nails may be caused by nail melanoma, which is mainly manifested as a longitudinal pigmented band with fuzzy edge, irregular and uneven pigmentation, which may be accompanied by symptoms such as ulceration, nail loss and nail destruction. It has a certain malignant tendency and can be treated with the help of conservative surgery plus whole skin transplantation and amputation surgery. 4. Nail biting fetish: nail biting fetish patients on the nails will involve the nail plate, proximal nail folds and other parts of the nail, thus causing damage to the nail matrix, secondary to the emergence of nail plate anomalies, such as fingernails on the emergence of fine black lines, etc., can be through the suggestion therapy and other psychological interventions to help treatment. It is recommended that patients with fine black lines on the fingernails go to the hospital, under the guidance of the physician for treatment, do not blindly use their own medication.