With the stochastic power fluctuation occurring on the ultra-high voltage（UHV）AC tie-line of North China power grid and Central China power grid，a control performance index，namely the multi-zone tie-line index（MT index）is proposed.The index is a combination of the basic ideas of both NERCs control performance standard（CPS）and standard T.This index focuses on the automatic generation control（AGC）of UHV tie-lines of interconnected power grids.It can be used to evaluate the subareas contribution（e.g.provincial power grids）to the control of power fluctuation on the main tie-line in a large power grid.Based on this，an AGC controller for suppressing the power fluctuation on main tie-lines is designed.Simulation results show that controllers designed under MT index can be used to suppress the power fluctuation on main tie-lines efficiently.