As the most dominative component under stress in an external combustion cam engine, the working condition of piston is very rigor. Once new design type and technical improvement is applied, it is necessary to analysis its thermal load and take secure steps. And the finite element model on each conditions of thermal is calculated, which is used to estimate the temperature field and provide a theoretical basis for further structural strength analysis and optimization design. Choosing analysis results of the piston as reference and taking five structural parameters of the piston as design variables, two objective functions including piston mass and maximal Von Mises stress are respectively considered. The optimum design of the piston is executed and the results indicate that it is feasible to improve temperature field and strength of the piston. These results enrich and develop the research on structural analysis and optimization of spatial engine, which are of guiding significance for analyzing engine strength and related problem in theoretically.