For interferometric testing of optics with surface qualities of approximately 1 nm, thermally caused deformation of mirror surface cannot be ignored. The various temperature conditions of interferometer environment are analyzed and temperature distribution of the mirror under test is simulated using I-Deas software. The temperature distribution was then used as thermal load of linear static analysis to perform mirror thermal deformation analysis. The peak-valley value and root-mean-square value surface figure error of a 300 mm convex lens caused by ambient temperature fluctuation is achieved. The environmental temperature fluctuation should be less than ±0.1 ℃, temperature difference be less than ±0.05 ℃ in mirror axis direction and ±0.1 ℃ in mirror diameter direction to perform optical test of 1 nm surface figure error, which is instructive to high quality optical testing.