Pre-pressurization rise process of the liquid oxygen stored in a tank is simulated through 3D CFD simulations. The phase transformation rate and temperature stratification of liquid oxygen are specified. The results suggest that evaporation rate of the liquid oxygen is fluctuating around 0.012 6 kg/s under the aerodynamic heating boundary condition. The liquid oxygen boiling rate near the wall is also investigated and averagely reaches 0.086 kg/s in the whole process, which is consistent with the tank heat leakage. The temperature stratification is observed when the high-temperature gas pressurization enters the tank by a diffuser. The liquid oxygen temperature in the gas-liquid interface increases from 89.51 K to 93.61 K. The average increasing rate of the liquid temperature is 0.022 K/s for 183 s after the pre-pressurization.