The black bruise in the nail cap of the big toe may be caused by nail fungus, gray nail and ingrown nail, or it may be caused by vitamin deficiency or trauma. It is advisable to go to the hospital for examination and targeted treatment after clarifying the cause.1. Nail fungus: Nail fungus may be caused by excessive trimming of the toenail or by Staphylococcus aureus infection, which usually manifests as redness, pain and swelling around the nail. When the nail tissue is swollen and darkened, there may be black bruises in the nail cap of the big toe, which can be treated by applying mupirocin ointment, fusidic acid cream and chlortetracycline ointment as prescribed by the doctor. 2. Grey nails: Grey nails, also known as onychomycosis, are usually caused by dermatophyte infections and may also be caused by inattention to local skin hygiene. The actual nail is usually the same as the nail that has been in the nail. You can take itraconazole capsules, terbinafine hydrochloride tablets and other medications as prescribed by the doctor, and in serious cases, you need to pull the nail; 3, ingrown nail: may be due to the toenail is too short, wear shoes too tight, etc., there will be local skin redness, swelling, pressure pain, abscesses, etc., abscesses more serious may appear in the big toe nail cover with black bruises like things. It can be treated with a nail plate brace under the guidance of a doctor. For patients with more serious disease, it needs to be treated by partial or total nail plate avulsion; 4. Nail mother nevus: If you have a nail mother nevus, there will be a pigmented nevus formed by melanocytes under the nail plate, and there will be something like black bruises in the big toe nail cap. Usually there is no need to intervene for treatment, but there is a certain chance of cancer, so close observation is needed; 5. Trauma: When the big toe is hit by violence, it will cause the capillaries to rupture and bleed, and the local skin will become swollen and painful, and when more bruises accumulate, there will be black bruises in the nail cap of the big toe. You can follow the doctor’s instructions to take blood circulation and pain relief capsules, ibuprofen extended-release capsules and other drugs for treatment.