For a specific frequency control system including AGC, there were relative differences in system frequency response due to the power disturbances in different frequency. The sensitive disturbances were defined in this paper by considering the natural characteristics of frequency control system including AGC. Establishing the frequency control system model, then analyzing its transfer function is an efficient way to obtain the system frequency response characteristics. Actually, it is hard to know the detail system model and the exact system transfer function, a method of obtaining the sensitive disturbances of frequency control system including AGC was proposed, in this method, the frequency-response trajectory in the condition of step load disturbance was needed, which is easy to be satisfied. The simulation example based on a simple frequency control system including AGC was given, analysis result show the validity of this method of obtaining sensitive disturbances by analyzing the frequency-response trajectory.