To predict the safety performance of fuel elements during operation of a space thermionic reactor, this study developed a mechanical performance analysis program for fuel elements, and conducted high-precision simulation of stress, strain, and geometric deformation for the multi-layer cylindrical TOPAZ-II thermionic fuel element. The program takes into account the irradiation swelling of nuclear fuel under high-temperature radiation environment, and analyzes the mechanical response of the fuel pellet-emitter after contact, thereby quickly and accurately solving the mechanical state of the fuel pellet and emitter, to provide accurate prediction of the performance during operation of the space thermionic reactor. The results indicate that in normal operating conditions, the fuel of a space thermionic reactor undergoes significant swelling effects, which causes deformation that can lead to potential safety hazards such as reduced thermal-electric conversion efficiency and component failure.