There do exist natural power fluctuations and the fluctuations caused by various disturbances on the alternating current（AC）tie-lines among control areas.But large-magnitude power fluctuations on the tie-lines may threaten the security of the system.Based on the practical data of power fluctuations on a tie-line between two large power systems，the frequency-domain analysis method and the autocorrelation analysis method are used to analyze stochastic power fluctuations of tie-lines.It is found that the component with the period about110s is prominent in the fluctuations.This is caused by the real-time area control error（ACE）regulation of the power system，i.e.the real-time ACE regulation can reduce the magnitude of the low-frequency slow component in the area control error，but it may amplify the area control error with the period around110s.The reason is verified with an experiment in practical systems.When the real-time ACE regulation stops，the component with the period about110s is significantly reduced while the low-frequency component increases.The study provides basis for analysis of tie-line power fluctuation mechanism and the improvement of tie-line control.