Abstract: Unit # 4 of a certain factory is a coal-fired thermal power unit, and the excitation system of the generator adopts the self parallel excitation method at the generator end. The excitation regulator adopts the NES 6100 digital excitation regulator produced by Nanrui Electric Control Company. Critical gain measurement and other tests were conducted under the operating conditions of 180MW (the declared lower limit of peak shaving power of the unit is consistent with the actual lower limit of peak shaving power) to verify and reverse adjust the damping effect of the set PSS. The on-site test results show that the optimized PSS has a good suppression effect on low-frequency oscillations during deep peak shaving of the unit, and the unit meets the standard requirements for reverse regulation under normal maximum load increase or decrease speed; When the active power of the unit reaches the lower limit of the actual peak shaving power of the unit, the PSS can be put into normal operation. Through this research and application, a PSS calculation model has been established in PSASP, which can be used for power system stability analysis and calculation, and has promotion and reference significance for the performance optimization of PSS in similar units。