To reduce the weight of the large-aperture (740 mm × 480 mm) rectangular scanning mirror and ensure the surface accuracy of the mirror, this study proposes a lightweight structure design with back openings, mainly triangular lightweight holes, and a lightweight rate of 81.4% by combining the two- and three-dimensional equivalent stiffness models. Based on the principle of ball joint and flexible hinge, two types of scanning mirror components with different three-point back support modes are designed. The finite element analysis results show that under the coupling condition of Y-direction gravity and a uniform temperature difference of 40 ℃, the root-mean-square (RMS) value of the mirror surface error of the ball joint and flexible hinge support structure are 12.3 nm and 12.9 nm, respectively, which satisfies the optical design requirement of less than or equal to 0.025λ (wavelength λ=632.8 nm). Furthermore, the first-order natural frequencies are 68.1 Hz and 85.5 Hz, and the flexure hinge structure had better stiffness. The surface error of the flexure hinge scanning mirror is measured using the autocollimation method. The measurement results show that the surface error RMS value is 0.025λ. This result provides a reference for developing a large-aperture rectangular scanning mirror assembly.