Excitation system based on full controlled devices converter(FCDES), has not only the ability of ordinary excitation system but can also supply more damping to the synchronous generator and increase the stability of power system through injecting reactive current to the grid. But as the increase of variables and the change of the system structure, the whole system becomes much complicated and harder to design the control scheme. For this complicated nonlinear system, this paper first analyzed that it's equivalent to compensating reactive power in grid side with the compensating shunt impendence. Based on this conclusion, this paper deduced a third-order model of this kind of excitation system. And with the main goal of controlling the rotor angle as well as the terminal voltage, a MIMO nonlinear control strategy was designed through this model. The simulation results show that, comparing to the linear control scheme (AVR+PSS), this nonlinear scheme has better effect in enhancing the system's damping, with the compensation capacity increase, and better effect can be achieved; when system parameters changed, still has a good performance and better suitability.