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For higher volumetric efficiency and lower launch cost of gear pumps with ultra low viscosity working medium used in spacecraft. In this paper, the volumetric efficiency formula of gear pumps was firstly derived from the two main leakages of axial gap and radial gap, At the same time, a new contact ratio factor formula appropriate for undercut gear and standard gear was proposed by the estimation of limited meshing point in driven gear or driving gear. When the undercut point of driven gear was being meshed, if the corresponding meshing pressure angle of driving gear was less than the addendum pressure angle, which showed that the addendum point of driving gear couldn't be meshed, the contact ratio of gear pair should be calculated by the undercut point position of driven gear; conversely, it showed that the undercut point of driven gear couldn't be meshed, the contact ratio should be calculated by the addendum point position of driving gear. Secondly, based on the newly created volumetric efficiency formula and contact ratio factor formula, and the optimal design method theory, with the 2 key parameters i.e. the minimum allowable volumetric efficiency of 90% and the ultra low viscosity value of 0.00018 Pa·s, the optimization design model was established, which took the minimum unit displacement volume being equivalent to the pump weight as the optimization goal. By the optimization of basis parameters of gear pairs including modulus, tooth number, addendum coefficient, pressure angle and displacement factor, the volumetric efficiency decline problem was consequently solved, and the minimum space launch cost of pump weight was guaranteed. Thirdly, the effects of structural parameters of gear pumps on the optimization result were analyzed. All optimization results showed that if the usual addendum contact ratio factor formula of driving gear was used for calculating contact ratio factor in the optimization design model, with the optimized basis gear parameters, the actual contact ratio factor was -0.3429, which was not reliable for undercut gear, and the continuous gear transmission also couldn't be guaranteed, so only the newly created contact ratio factor formula was reliable; the influence degrees of the pump parameters including pressure angle, shaft radius, tip radius of rack cutter, and starting angle of transition region on the optimization result were respectively 6.07%, 7.8%, 2.9% and 6.4%. On the whole, these parameters had little influence on the optimization result; especially, the free space was endowed to the structure design to reduce the radial force. But the influences of radial gap and axial gap on the optimization result were very large, and unexpectedly, there was no optimal solution with the axial gap value of 0.06 mm, so the upper limit values of radial gap and axial gap could be determined by the influence turning points. For example, the upper limit value of axial gap was 0.04 mm, and the upper limit value of radial gap was 0.07 mm. For other pump parameters, there were such turning points. For example, the upper limit value of shaft radius was 13 mm, and the upper limit value of starting angle of transition region was 180°. As a whole, the designed pump shape had a small axial size and a large radial size. Finally, the first attempt of the optimal gear pump design for spacecraft clarified that the ultra low viscosity medium was also applicable to gear pumps, and catered to the development of Chinese aerospace industry. The study provides a demonstration of the optimal design method for other hydraulic pumps with ultra low viscosity medium as well. [ABSTRACT FROM AUTHOR]