In the early stage of nail nevus, very thin longitudinal lines appear on the nail, which may be brown, gray or black. As the nail nevus gradually grows, the color band on the nail becomes wider and wider, and even the whole nail turns black. Early nail nevus is difficult to distinguish from nail bleeding, nail freckle-like nevus, nail fungus and early nail malignant melanoma. In children, the chance of malignant nail nevus is very low. In adults, if the width of the color band exceeds 3mm, the color band widens rapidly within a short period of time, the color of the band is uneven, the boundary is unclear, the nail plate is damaged and the skin around the nail is involved, the nail melanoma is alerted and needs to be examined and treated.