In daily operation, taking Xiandawang Mine Industrial Plaza sewage Treatment Plant as an example， the sudden change of influent ammonia nitrogen load often occurs， for example, the variation range of influent ammonia nitrogen in Xiandawang Mine Industrial Plaza sewage Treatment Plant could reach about 35 mg/L, and the removal rate of ammonia nitrogen will be low if the traditional operation mode is adopted, and the effluent water quality may exceed the standard. According to the principle of biological removal of ammonia nitrogen, the flexible operation mode of CASS process can be fully utilized to strengthen the role of nitrification and denitrification and improve the removal rate of ammonia nitrogen, which can not only ensure the quality of effluent water, but also reduce capital investment. The experiment showed that the new operation mode can greatly improve the removal rate of ammonia nitrogen, which could be popularized and applied in similar processes.