To analyze the influence of temperature on the large aperture light weight mirror reflecting surface, a three-dimensional model has been established based on a large aperture light weight mirror. The steady-state and transient thermal characteristic is studied by taking the advantage of Finite Element Method (FEM) software patran/nastran. The uniform temperature distribution, axial temperature difference, radial temperature difference and the changing ambient temperature have been analyzed. The analysis shows that in the case of axial temperature difference, radial temperature difference and the changing ambient temperature, the mirror surface suffers serious thermal aberration, and the PV value and RMS value increase linearly with the temperature gradient. The influence from uniform temperature is slight, at the temperature of ±3 ℃, the PV value and RMS value are respectively 27.6 nm and 6.5 nm with defocus aberration removed.