This paper investigates the adaptation of PSS under different running conditions. The principles of Prony are firstly studied, and the accuracy of this method for the engineering calculation is also confirmed by an example. Then based on the PSD-BPA simulation computing, the vibration characteristics of the generator set, and the adaptation of PSS with the variations of the generator load factor, the grid load and the grid structure, are respectively analyzed. The corresponding suggested flow chart of the PSS parameter setting is proposed as well. It is shown that, as the generator load factor or the grid load is increased, the vibration frequency will be decreased. However, there are no evident rules for the frequency change when the grid structure is changed. As the generator load factor rises, the adaptation characteristic will be firstly debased and then optimized, while the increment of the grid load will result in an opposite result. The achievements obtained in this paper can be used as a reference for the PSS parameter setting improvement.