Automatic generation control（AGC） system is used to achieve load and generation balance by maintaining the system frequency and the tie line flows at their scheduled values. In order to mitigate the power fluctuations on the ultra-high voltage（UHV） tie-line， a coordinated control strategy between regional and provincial regulating zones is presented. In this strategy， the outputs of regional and provincial units are adjusted according to the power deviation of the UHV tie-line. Under normal conditions， the regional units act to mitigate the power fluctuations on the UHV tie-line when the power deviation is large and reset to scheduled value in advance when the power deviation is small. A compensation control strategy is proposed as well， in which the regional regulator is driven to balance the load disturbance if the AGC capacity of provincial regulator is insufficient. The numerical simulation results show that the proposed method works well under normal and emergency conditions of UHV AC interconnected power system.