Frequency domain analysis and autocorrelation coefficient analysis show that the component with a period about110s is prominent in the stochastic power fluctuations of AC tie-lines between two large interconnected power systems.The mechanism model reflecting the operation principles of the actual automatic generation control（AGC）system is constructed to simulate the process of tie-line power control.The cause of the prominent component is revealed by simulation result analysis，and the effect of AGC on the power fluctuations of tie-lines is comprehensively analyzed.Based on the analysis，measures to improve the tie-line power control by optimizing the parameters of AGC systems are proposed.This is achieved by adjusting the filtering time constant of area control error（ACE）as well as reducing the control delay.The approach is verified by simulations.The results are helpful for better control of tie-line power.