Abstract: SCR system has the features of large delays and inertia which makes it difficult to realize the accurate control of ammonia injection based on traditional PID method during the process of start and stop of coal mill.In view of this problem,start and stop signal was selected by analyzing of operation data to build feedforward compensator for ammonia injection and SCR ammonia injection optimal control strategy was proposed based on cascade control method.The effect of the control strategy was confirmed through constant value disturbance test,variable load test and coal mill load test.The results show that ammonia injection optimal control strategy solves the problem of NO_x concentration exceeded discharge standard during the process of start and stop of coal mill,control performance of SCR system is improved and the timely and accurate control of the amount of injecting ammonia is achieved.Optimization control strategy has good performance on the efficiency of denitration and avoiding excessive injection of ammonia.Both of the dynamic and static regulating quality of SCR system could meet the relevant national standards.