According to the structural design requirements of a 700 mm×249 mm rectangular space mirror assembly, the mirror and its support structure were designed in detail. First of all, from the perspectives of material selection, diameter-to-thickness ratio, support scheme and lightweight form, the structure design of the mirror was carried out. The number of support points of the rectangular mirror was obtained by theoretical calculation. The support points were optimized, and the influence of the support holes on the deformation of the mirror's weight was explored. Secondly, in order to meet the requirements of the force and thermal environment adaptability of the mirror assembly, a new type of flexible support structure was designed, and the influence of the weak link of the flexible hinge on the surface accuracy of the reflector was proposed; the position of the support structure was optimized, and the change rule of the reflector shape accuracy with respect to the position of the support structure was proposed. Then the finite element analysis of the mirror assembly was carried out. Under the load conditions of self-weight and 5 ℃ temperature rise, the maximum Peak Value (PV) and Root Mean Square (RMS) of the mirror surface reach 58.2 nm and 12.3 nm; the first-order natural frequency of the mirror assembly is 259 Hz, and the maximum stress response of the flexible support under the condition of low-frequency sinusoidal sweeping vibration is 138 MPa. Finally, a kinetic test was carried out. The test results show that the first-order natural frequency of the mirror assembly is 255 Hz, and the finite element analysis error is 1.7%. Analysis and test results show that the design of the mirror assembly is reasonable and meets the design index requirements.