The unsteady flow of the hydrodynamic coupling was calculated by using the sliding mesh method and mixture model in CFD software. And the velocity and pressure distribution of the hydrodynamic coupling were figured out. Through analyzing the velocity and pressure field, the distribution of the two-phase flow under different filling rate were summarized. The torque was computed based on the numerical solution of 3-D flow field, and the original characteristic of hydrodynamic coupling was predicted. The predicted characteristic was compared with the experimental data. It indicates that the computational result of numerical simulation is well tallied with the results of the experiment, and the result can reflect the basal characteristic of the inner flow in hydrodynamic coupling.