There are many reasons for the late stage of breast cancer to break down and bleed, including tumor cell proliferation causing tumor necrosis, ulceration and bleeding, local trauma or local infection and so on. 1. Tumor necrosis, ulceration and bleeding: late stage breast cancer can be manifested as tumor necrosis and ulceration in the breast, bleeding and breaking to the skin, and it can also lead to breaking and bleeding of lymph nodes around the breast, which is due to the fact that the tumor cells of the patient with breast cancer shed and invade the surrounding lymph node tissues, and the massive growth and reproduction of the tumor causes the tumor ischemia and breaks down. 2. Local trauma: firstly, we can monitor the vital signs, clarify the amount of bleeding, and further complete auxiliary examination to clarify the location of the tumor, and then we can adopt radical treatment such as radical tumor surgery. 3. Local infection: tumor necrosis is easy to be infected, which can lead to purulent inflammation, causing breast cancer ulcerating and flowing, accompanied by bad smell, and anti-infection treatment can be carried out according to the pathogenic bacteria. It is recommended to go to the hospital in time to find out the cause of breast cancer, actively treat it, keep a good attitude and establish confidence in treatment.