The discontinuous thrust settling approach will be adopted in China to extend the coast time of cryogenic upper stages， meet the needs of deep space exploration and improve the adaptability of launch vehicles. The major difficulty is the propellant reorientation process. As current reorientation simulation is mostly two-dimensional computational fluid dynamics （CFD） simulation and cannot accurately and reasonably predict the bubbles escape process， this paper proposes a three-dimensional CFD simulation method based on Flow-3D. This method uses the ratio of the volume of the entrained gas to the volume of the fluid to predict the bubbles escape process. It is more accurate and reasonable than the previous one that uses the number of bubbles to excert prediction. The simulation results of reorientation and bubbles escape process are consist with the results of the low-gravity reorientation in a scale-model centaur liquid-hydrogen tank， and the error at the corresponding time is no more than 10%. The simulation results show the feasibility of the discontinuous thrust settling approach for propellant management， and determine the relevant parameters of a certain upper stage.