The Sum of Squares robust control approach (SOSRCA) is applied to the excitation and steam-valving coordinated control system with uncertain disturbance. The SOSRCA model of the control system is built. The control strategy fully considers the uncertain parameters and interferences existing in the control system, and the robust performance of the generator set system is good. The solution process of the control strategy is algorithmic and programmatic, which simplifies the design process and reduce the complexity of control. The simulation model test of the SOSRCA law is carried out in a three-machine power system. The result verifies its effectiveness and superiority.