To improve the volume rate of external gear pump with lower viscosity medium used in spacecraft and reduce the pump weight for launch cost down,the formula of volume rate was derived from the two main leakage of axial and radial,and the double objective optimization model with minimum pump volume and maximum volume rate was constructed.Based on three kinds of optimization schemes,which are only considering the minimal internal leakage,considering the minimum pump volume and minimum internal leakage synthetically,and only considering the minimal volume,the optimization was carried out respectively and the influence of relevant parameters was analyzed.The results indicate that,in order to adapt to the lower viscosity medium,gear addendum circle diameter and gear addendum thickness of the pump shall be as large as possible,the axle hole diameter as small as possible.The axial gap has enormous influence on pump volume rate,and it is the key element for reducing the internal leakage.The best choice for increasing the volume rate is to use full tooth seal without measures reducing the radial force in the transition zone.The larger inlet and smaller oil outlet diameter,is conducive to improve the pump volume rate,and avoid pump cavitations due to the insufficient suction.