Based on the application of phase change thermal storage in the field of aerospace, physical and mathematic model of the tubular regenerator was built. Numerical simulation was adopted to study the heat transfer characteristics under different gravity and Reynolds number. A topology optimization method based on variable density method was introduced to optimize the structure of a regenerator. A long range topology fin model was designed, and the influence of various factors on the melting rate of phase change material (PCM) was compared. The results show that the melting rate of PCM under the same Reynolds number is slower than that under the gravity condition, and the melting rate under the different Reynolds number is 82.7%, 86.1% and 90.1% lower than that under the gravity condition, respectively. However, increasing Reynolds number is not enough to effectively improve the melting rate of PCM under microgravity conditions. Compared with the unfinned tube, the heat transfer enhancement effect of the fin model obtained by topology optimization is obvious, and the melting time of the phase change material is reduced by 47.83% compared with the melting time of the unfinned tube under microgravity condition, indicating that the topologically optimized fin model can effectively reduce the influence of microgravity on the heat storage and release rate.