Abstract: The upper stage tank is in a microgravity environment during the inertial flight. If the liquid propellant in the tank is not well managed, the normal restart of the main engine would be affected. In this paper, the Volume of Fluid method is adopted to simulate the free interface deformation of the propellant in an upper stage tank under the microgravity condition, and the results are compared with the theoretical analysis results. It is shown that the liquid climbing height and the surface tension wave propagation velocity obtained by the numerical simulation are in good agreement with the values from the fluid mechanics theory, indicating that the calculation method can be used for predicting the fluid motion characteristics of the upper stage liquid oxygen tank under the microgravity condition, to provide a reference for the propellant management of the subsequent upper stage as well as a support for the related test verification work.