The main reason is that chemotherapy destroys the immunity of the body and causes an imbalance in the immune system, which is reflected in the mouth ulcers. Oral ulcer is the most common oral mucosal disease in dentistry, which has typical recurrent and self-limiting characteristics. For self-healing mouth ulcers, they usually recur after a period of time. Moreover, mouth ulcers will heal naturally in 1-2 weeks even without special treatment. In addition, mouth ulcers are now considered to be caused by a variety of factors, but one of the more important factors is the immune factor, and immune imbalance has been identified as a cause of mouth ulcers. In addition to the damage to the oral mucosa caused by chemotherapy drugs, chemotherapy itself has a large impact on the immunity of the human organism and is also a cause of mouth ulcers.