This paper proposes an improved synergetic excitation controller (ISEC) for synchronous generator in order to not only improve transient stability but also obtain good voltage regulation performances of power systems. According to the control objective, a manifold is chosen as a linear combination of the deviation of generator terminal voltage, rotor speed and active power for the synthesis of the synergetic excitation controller. Then the control law of the conventional synergetic excitation controller (CSEC) is deduced based on the nonlinear model of the synchronous generator. The influences of controller parameters on control performance and its selection principle are also discussed. Compared with CSEC, an adaption strategy is proposed for ISEC to vary the control parameter in order to improve the performances of the voltage regulation and transient stability under various operating conditions. Case studies are undertaken on a single-machine infinite-bus (SMIB) power system. Simulation results show the ISEC can provide better damping and voltage regulation performances in comparison with the CSEC and conventional power system stabilizer.