The differential equation of heat transfer for a space point target is established, and the theoretical temperature change of the point target is herein calculated. Then, for the inflatable sphere in space with specularly-reflecting or diffusely-reflecting inner surface, by considering the multi-layer ultra-thin materials as equivalent to a single-layer material, the unsteady heat transfer equation of micro-facets is established for calculating the superficial temperature distribution of the sphere and inducing its equivalent radiation temperature. The calculation results show that the equivalent radiation temperature of the target is consistent with the theoretical temperature change of the point target. The specular or diffuse reflection characteristics of inner surfaces do not affect the equivalent radiation temperature of the target. Influenced by the radiation transfer coefficient, the superficial temperature difference of the sphere with specularly-reflecting inner surface is larger than that with diffusely-reflecting inner surface, and the lower the emissivity, the more obvious it is. When the emissivity is close to 1, the superficial temperature distribution in both cases tends to be consistent.