The invention discloses a microstress assembling method for a large-caliber reflector assembly. Related devices comprise sensors, supporting rods, weights, a base, pulling ropes and a tray. Firstly, the mechanical sensors are fixedly installed on the base; secondly, the reflector assembly is placed on the base through three ball head gaskets; thirdly, the supporting rods are installed at the designated positions of the base, one end of each pulling rope is bound to a disassembling point, the pulling ropes wind around two fixed pulleys on the supporting rods, and the weights are bound at the other ends of the pulling ropes. At the moment, whether gravity disassembling is in place or not is judged through reading of the mechanical sensors, and the weight of the weights is adjusted in a fine mode until the reading of the sensors is zero. Finally, after disassembling is completed, a torque wrench is used for screwing screws of the reflector assembly to the preset torque, and disassembling and assembling of the reflector assembly are completed. Thus, microstress assembling of the reflector assembly is achieved, and the reflector surface shape quality and the structural stability of an optical remote sensor are ensured.