Abstract: The main steam temperature control system of an ultra-supercritical unit is a typical thermal system featuring large time lag, multi-perturbation, and nonlinearity. In the deep peaking condition, the boiler combustion stability decreases, the hydrodynamic force is insufficient, and the ecological migration of radiant/convective heat exchanges; thus it is difficult for the traditional PID serial control to satisfy the higher control demand. In this paper, a main steam temperature cascade control system based on internal mode control and improved self-impedance control is proposed. First, for the nonlinear and hysteresis characteristics of the inner loop, the nonlinear self-oscillating controller is improved and applied. Then, for the large delay and inertia characteristics of the outer loop, the inner mode control is used to predict and regulate ahead of time to enhance the tracking ability of the outer loop to the set value and the robustness of the system. Simulation shows that compared with the traditional PID series control, this method has better tracking, disturbance resistance and stronger robustness for the main steam temperature control object. Practical application in a 1 000 MW unit shows that the method can adapt to the complex and changing environment, meet the "two rules" assessment requirements of the power plant, and effectively improve the main steam temperature control characteristics, with the steady state deviation of steam temperature controlled in ±2℃, and the deviation of variable load ≤±5℃, which has a good prospect for engineering application.