In this article, a unit heat pipe receiver in an advanced solar dynamic system is numerically simulated. Accordingly, a mathematical model is set up, a numerical method is offered, and numerical results are compared with numerical results of the National Aeronautics and Space Administration (NASA). With both void cavity and phase change considered, influence of radial thickness of phase change material (PCM) on thermal performance of heat pipe receiver is numerically analyzed under microgravity. Numerical results indicate that decreasing the radial thickness of PCM in each canister has an effect of decreasing the thermal resistance of PCM. The variation of heat pipe temperature decreases as the thermal resistance between the heat pipe and the melt front decreases. The variation in heat pipe temperature is seen to be substantially reduced as the radial thickness of PCM is decreased to between 14.5 and 17 mm. Beyond this point, the impact of further reductions appears limited. The effect can be achieved with far fewer canisters with some degree of increasing thermal enhancement. When the radial thickness of PCM for unit heat pipe receiver is between 14.5 and 17 mm, the thermal performance of the heat pipe receiver is stable, and thermal spot during sunlight periods and thermal ratcheting during eclipse periods may be alleviated. The research results can be used to guide the designing and optimization of a PCM canister for a heat pipe receiver.