Taking the compressibility and continuity characteristics of liquid into account, the dynamic model of distributing valves was constructed, and the volumetric efficiency character was studied. When the working frequency was approximately to the round number times of the natural frequency of valves, the efficiency was seriously low. The nominal rotational speed of the prototype was close to the speed where the volumetric efficiency was the lowest, which was harmful to the normal work of ICP. The change of volumetric efficiency has great influence on the fuel-consumption, output flow, effective thermal efficiency, effective power and so on, but little on the output pressure. 