For the application requirements of power system stabilizer for generator excitation system, the functional requirements of PSS devices are analyzed. And a new external power system stabilizer device based on PowerPC + DSP as the control core is proposed. The hardware and software design of the device is described, and a real-time measurement method of the generator rotor speed is proposed. The efficiency and applicability of this device are demonstrated by dynamic simulation test. The device provides a way to realize the transformation and upgrading of PSS for excitation system through the external device, which can enhance the effect of the generator on suppressing the low frequency oscillation of the power system and improve the stability margin of the power grid.