Abstract: In view of the process of helium pressurization in a liquid oxygen tank during outflow,the numerical methods of zero-dimensional integrated model,one-dimensional stratification model and two-dimensional computational fluid dynamics(CFD) model were adopted to study the variation law of ullage physical field,respectively.An empirical formula was set up to solve the heat transfer rate between ullage and tank wall in the zero-dimensional model and in the one-dimension model,while the low Reynolds number k-ε model was selected to calculate the coupled heat transfer rate between fluid and tank wall in the two-dimensional CFD model.The helium density was calculated by the ideal-gas model.The ullage pressure,ullage average temperature and temperature distribution were forecasted by these three models,respectively.The computational results show that the forecasted ullage pressure and ullage average temperature of three models show reasonable distribution,and the results of different models can be confirmed one another.In addition,the calculated axial temperature distribution in the ullage gas and in the ullage wall of the one-dimensional model is close to that of two-dimensional model in the main region,while the forecasted temperature distribution inside the ullage top nearby gas injector obviously deviates in different models.Moreover,the higher inlet velocity of the pressurized gas means more obvious radial temperature distribution inside tank top.Thus the two-dimensional CFD model should be selected to present the physical field distribution in the ullage when the inlet velocity is high.