Advanced Stirling convertor designed by Sunpower was investigated as the object of the study, and a theoretical model of a Stirling cycle was established. Based on the operation condition of the nuclear power plant in the space, the theory of finite time thermodynamics was adopted to investigate the system performance, considering the effect of variable heat absorption temperature on the cycle. Moreover, applicability of the system optimization based on the entropy theory and the entransy theory was discussed. In addition, results between the ecological optimization criterion and the output power optimization criteria were compared. It was found that cycle efficiency and output power could be effectively improved through increasing the cycle endothermic temperature. In addition, the output power could be optimized by using entropy production rate and entransy loss rate. The output power optimization criterion can be applicable at lower exothermic temperature condition, while the ecological optimization criterion is more reasonable at higher exothermic temperature condition.