The back of the neck has a hard lump commonly due to the following reasons: First, sebaceous cyst formation, the disease is caused by follicle obstruction, often due to repeated local friction, sweating and so on, usually need to go to the hospital for ultrasound to make it clear that the disease needs to be performed surgery to be able to get the root cause of the disease, otherwise it will lead to local redness, swelling, and ulceration, and may bleed pus. Second, osteophytes, because the back of the neck is the cervical vertebrae, osteophytes can be manifested as a localized hard swelling, which can be performed cervical spine X-rays, CT, etc. to clarify. Third, local mosquito bites. Fourth, local lymph node enlargement, when the lymph nodes are enlarged, you can touch the local hard swelling, sometimes accompanied by pain, local skin redness and swelling. Fifth, there are other rare causes, such as allergies or autoimmune diseases. In conclusion, if the abnormality persists, it is necessary to go to the hospital in time to clarify the cause of the disease.