The transient unsteady two phase flow of high-power variable speed hydrodynamic coupling was numerical simulated based on the 3-D multi-phase flow theory and computational fluid dynamics (CFD) by using the sliding mesh method and the characteristics was predicted. The pressure and velocity distribution were analyzed to reveal the flow rule and characteristics of flow field. The reduced efficiency reasons were revealed. The theoretical calculation was compared with the external characteristic experiment under different filling rates. The maximum error was less than 15% between computational result of original characteristics and experiment results, and the correctness of theory method was verified.