Abstract: In order to enhance the effect of secondary power frequency control in large power system,the paper studies the effect of automatic generation control(AGC) on power frequency based on the full dynamic simulation. First modeling of AGC for the full dynamic simulation according to the three components of AGC ,which is calculation of area control error (ACE), simulation of control strategy, and calculation of generating power regulation. Second the effect of key-parameters of AGC including dead band, transient frequency-deviationon and rate of generating power regulation on power frequency is studied by simulating.Finally, the paper takes Fujian power as an example to analyze the effect of AGC on under-frequency loading shedding(UFLS) by the serious faults and study on the coordination control between AGC and UFLS.