A black line in the middle of the big lesser toenail of the foot may be caused by the lack of micronutrients, subnail pitting hemorrhage and nails mother mole, which are analyzed as follows: 1. lack of trace elements: foot big lesser toenail has a black line if it is relatively shallow black streaks, may be due to the lack of trace elements, can be supplemented by trace elements, can improve this phenomenon. 2. Under the nail point-like bleeding: when accidentally toenail bumped into a place, there may be point-like bleeding, the formation of a black or purple-black thin line, usually relatively short. It does not matter, it will gradually move out with the growth of toenail and return to normal gradually. 3. Mole of the mother of the nail: a sudden pigmented mole at the root of the nail, may be a melanoma with the growth of the toenail gradually formed a black vertical stripe, generally difficult to subside, if the black line becomes thicker in a short period of time or suddenly become black, or in the root of the nail on the skin of the emergence of a number of small moles, you need to go to the hospital in a timely manner to perform surgery combined with doing dermatopathology. A black line in the middle of the big lesser toenail of the foot is also seen in other causes, should consult a doctor in time, under the guidance of a professional doctor to standardize the treatment, so as not to delay the condition.