A reflective optical system is designed for arcsecond and higher precision star sensors to eliminate the chromatic distortion. Compared to traditional refractive systems, the optical axis stability of the reflective system is more susceptible to thermal deformation caused by the ambient temperature, which in turn causes the optical axis pointing drift. The flexible structure has the effect of isolating the additional load, which can improve the optical axis stability. In this paper, the performance of two types of flexible structures, Bipod and Groove, are compared under different types of temperature fields based on multidisciplinary simulations to investigate in detail the resistance of the two structures to different loads, and the substitutability and combinability between the two structures. The results show that both flexible structures can improve thermal stability of the optical axis by a factor of two to five, and the effect of the Bipod structure is more significant. The thermal stability can be further improved by up to one order of magnitude with a proper combination of two structures, which contributes to the development of the next generation star sensor with extremely high precision.