The Central China Power Grid (CCPG) and North China Power Grid (NCPG) are connected together by a single AC ultra-high-voltage (UHV) transmission link. When suffering power disturbances, power oscillation with high magnitude and long period would happen on the UHV tie-line, which may affect the synchronous and stable operation of the interconnected two power grids. This study proposes a novel emergency control scheme for high-voltage direct current (HVDC) in CCPG to suppress the high peak value of the power oscillation. Based on a two-machine equivalent model, the maximum rising slope of power oscillat`ion is used to predict the peak value of power oscillation and power disturbance which is used as the reference of emergency power modulation. Simulation verification is carried out based on the full model of the CCPG and NCPG, using Power System Analysis Software Package. Moreover, field test has also been carried out in CCPG. Both simulation results and recorded wave curve show that the proposed HVDC emergency control scheme can effectively suppress the peak value under different power disturbances. This significantly improves the transient performance of the interconnected power system and the power transfer capacity of the UHV tie-line.