The equivalent stress method was proposed to analyze the surface change of the mirror under the temperature change. The forced displacement restrain was applied on the bonded position of the mirror to ensure the mirror suffer the equivalent stress compared with the heat stress. The surface change of the mirror under the action of the forced displacement was consistent to the one under the action of the temperature change. A bonded experiment to the mirror whose diameter is 70 mm and thickness is 15 mm and surface accuracy of the mirror is measured by the fine ZYGO interferometer in different temperatures. It is indicated that when the temperature descended from 20℃ to 16℃, the mirror surface change of the Peak to Valley(PV) and Root Mean Square(RMS) are 0.005λ and 0.002λ, while the simulation change of the PV and RMS are 0.004λ and 0.002λ. When the temperature increased from 20℃ to 30℃, the mirror surface change of the PV and RMS are 0.013λ and 0.006λ. The simulation change of the PV and RMS are 0.012λ and 0.004λ. The experimental result agrees with the simulation basically, which illustrate the equivalent stress method based on the forced displacement restrain can express the effect of the temperature change on the surface accuracy change of the mirror perfectly.