For understanding the impact of high-speed centrifugation caused by rotating hydraulic oil on volumetric efficiency and phenomenon of trapped oil in external spur-gear pump, the amendment to original gas volume percentage mixed in inlet working oil was presented for analysis of this impact. From the assumption on only considering the compressibility of oil mixed with gas, and pure hydraulic oil and mixed oil respectively accumulated in outside and inside of non-tooth space, the formula of volumetric efficiency only under the impact of centrifugation , gas volume percentage mixed in trapped oil and amendment to original mixed gas volume percentage were derived. The simulation results of two sets of data showed that the impact on phenomenon of trapped oil under centrifugation was opposite to volume efficiency, the higher rotate speed, the smaller volume efficiency; the lower outlet pressure, the better effect of centrifugation on phenomenon of trapped oil. These indicate that the general parameters of gear pair in external spur-gear pump should be optimally designed, and the method can provide a reference for high-speed development of external spur-gear pump.