Many controlled objects have large inertia, large time delay and parameter time-variance features in a thermal power plant. A decoupled internal model controller program is designed for these controlled objects. This program is to combined decoupled with the controller as a complete controller to control the controlled objects. Through the decouple analysis and steady-state error analysis, the expression of the decoupled internal model controller can be derived. The simulation results show that this controller control system has good decoupling capability and strong robustness, with a good prospect of application.