In allusion to the structural characteristics of UHAC/UHDC hybrid, the influence of emergency DC power support (EDCPS) on secure and stable operation of actual grid is studied. The effects of power upgrade strategy on system stability are analyzed based on PSD-BPA to obtain the proper starting-time, the proper amount and the proper duration of power support. The power backdown strategy is adopted during swing-back process to improve the results of emergency power support. Contraposing the active power modulation strategy, the selection principle of input signals for DC auxiliary controller is put forward. The effectiveness of active power modulation and power upgrade (or backdown) as emergency power support strategies is analyzed by comparison. Simulation results indicate that active power modulation improves the stability margin and damping of following-swing and has better results of emergency power support. Study results are available for reference in the drafting of additional stability function for actual AC/DC hybrid power grid.