Abstract: AC tie-line with long distance is the weakness of synchronous stability. For AC synchronous grids with AC interconnection, large disturbances in a certain area will not only impact the stability of the grid in this region, but also have an influence on the stability of other regional grids through inter-regional tie lines. Taking North China-Central China interconnected power system as the research objects, the characteristics of power fluctuation on UHV AC tie-line after power impacts were studied. Fast calculation algorithm for maximum transmission capacity of power oscillation on AC tie-line after power impacts was proposed. The relevance of the electrical quantities during the first swing and the transient stability of interconnected power system were clarified, and on this basis, the emergency power control with UHVDCs to improve the transient stability of power system was put forward. The correctness of the identification criteria and the effectiveness of the DC emergency power control were verified by simulation analysis of interconnected power system.