Abstract: Aiming at the aluminum alloy material,a largeaperture primary mirror of infrared camera was designed with the mirror aperture of 420 mm. Starting from the diameterthickness ratio, the number of the support points and the lightweight structural form, a structure of open back, triangle lightweight construction and back threepoint support was designed. Dynamic stiffness of the mirror and surface shape change under the weight and temperature loads were analyzed by the finite analysis software. The results show that the natural frequency of the mirror is enough, which has a sufficiently high dynamic stiffness. Both surface accuracy RMS of the mirror(30 nm,0.2 nm) have reached the index requirements of the optical system(/10,=632.8 nm) under gravity and a uniform temperature drop of -40℃，which provides a theoretical basis for the design of infrared aluminum mirror.