The primary mirror is a key component of large-scale space telescope, and its thermal effects analysis is an important guideline to thermal design and control of optical system. Based on Finite Elements Analysis (FEA) method and thermodynamics coupling theory, the heat-induced distortion and the effects of temperature distribution such as edge, axial and radial temperature grads acted on the primary mirror are analyzed in this paper. According to the design requirement and the numerical simulation results, the effective means of thermal control for the primary mirror are suggested.