Abstract: For frequency oscillation, the oscillation strongly related to the process of automatic generation control（AGC） is called AGC oscillation. Based on the single-unit single-load system, the AGC oscillation is analyzed using open-loop transfer function.And the influences of various parameters or elements on AGC oscillation are studied by analyzing their influences on frequency characteristics of the open-loop transfer function. The results show that the stability margin of the open-loop transfer function of the system will be too small or even negative, thus reducing the stability of the system and even causing AGC oscillation if the frequency response coefficient and integral coefficient of AGC parameters are too large, the transmission delay of AGC is too long,and the amplitude or phase lag of the frequency characteristic in the generation unit is too large. The influences of the unit characteristics on AGC oscillation are also studied. When primary frequency regulation works, the amplitude of the unit will be smaller, thus improving the stability of AGC mode. The phase lags of prime movers for different hydropower or thermal units are different, which will affect the phase margin of the open-loop transfer function, and then affect the stability of AGC mode. The opening regulation mode tends to have faster primary frequency regulation speed than power regulation mode. Therefore, when adopting opening regulation mode, the unit will have lower amplitude at the oscillation frequency to make the AGC mode more stable. The results of theoretical analysis are all verified by eigenvalue analysis. Moreover, in the example of a multi-unit multi-load system, the conclusions based on the single-machine single-load system are verified.