The stability and control of power systems remain a challenge as the modern power system becomes more complex. A flexible excitation system (FES) based on fully controlled devices for the synchronous generator that can provide field voltage regulation and extra reactive power compensation, while the conventional half-controlled static excitation system (CSES) can only regulate field voltage for modern power systems stability enhancement. This paper applies the FES to the synchronous generator and proposes a nonlinear coordinated control strategy to enhance the voltage and rotor angle stability. To deal with the control of the nonlinear system which contains the synchronous generator and FES (SG-FES), the system is linearized to a two-input two-output system via partial feedback linearizing control (PFLC). With PFLC, the system is divided into a reduced-order linear part and a nonlinear dynamic autonomous part. The control method for SG-FES is tested with a single machine infinite bus (SMIB) system under a three-phase short circuit fault and small disturbance. The results show the effectiveness and better performance of the proposed control strategy compared with SG-CSES with conventional power system stabilizer, and avoid the need for rotor angle measurement compared with SG-FES with feedback linearization control.