Abstract: The large aperture fast steering mirror(FSM) are often used in the fields of space optical communications and laser weapons. The large aperture fast steering mirror surface figure test system with an effective aperture of 400 mm has a working wavelength of 633 nm. The system is mainly composed of an off-axis beam expanding system and a focal plane system to achieve real-time detection of the mirror surface figure error when the large aperture FSM is working. In this paper, the design parameters of the optical system and the selection of optical element parameters were described. The optical system was designed and simulated, and the effect of temperature variation on the optical system was obtained based on the integrated optical-mechanical-thermal analysis. The test results show that the large aperture fast steering mirror surface figure test system can achieve real-time recording and high precision measurement, meanwhile, the system has stability in a working environment with temperature changes. The stability of the surface figure test system is 0.048λ (RMS, λ=633 nm).