To control a chaotic phenomenon in the ninth-order power system, a new method for controlling the chaos of the power system is proposed. This combines PI control and fixed-time terminal sliding mode control based on a finite-time observer. The finite-time observer can predict the external disturbance, and the addition of the integral can eliminate the steady-state error of the system. The fixed switching control term of the traditional sliding mode is improved by adding an adaptive law to further enhance the robustness of the control method. The tanh(x/τ) function is used to replace the sign function of the traditional sliding mode and a new convergence law is used to solve the chattering phenomenon in the sliding mode control. Finally, in order to reduce the control parameter search time, a new objective function is constructed based on an improved ant-lion algorithm to find the optimal control parameters. The results show that the proposed control method has good control performance. This work is supported by the National Natural Science Foundation of China (No. 51967013).