This paper investigates the influences of operation conditions change and engineering equivalent on the uncompensated phase-frequency characteristics measurement of excitation system. Firstly, the principles of PSS increasing damping and the engineering equivalent measurement method of phase-frequency characteristics are elucidated. Secondly, based on theoretical analysis of single-machine infinite bus system, it is concluded that the active power and reactive power of generators are the main factors that influence the measurement of uncompensated characteristics. Then the actual power grid simulations are made, drawing a conclusion that the uncompensated characteristics are mainly affected by the active power and reactive power of the generator. Power grid load, electric distance, number of parallel units, and whether the PSS of parallel units are on or out have less influences on test results. Finally, through the simulation comparison, it comes to a conclusion that employing terminal voltage as the equivalent of internal electromotive force will cause the rear half band lag angle get larger and replacing the actual grid with a single-machine infinite bus system will make the full-band lag angle get smaller. The achievements obtained in this paper have a positive significance to improve the performance of PSS. This work is supported by National Natural Science Foundation of China (No. 51777074), Natural Science Foundation of Hebei Province (No. E2015502013), and Fundamental Research Funds for the Central Universities (No. 2018YQ03).