The invention discloses a support method for the mirror-image machining support side of a large thin-film member, belongs to the technical field of machining application for industrial robots, and relates to a near-constant-stiffness support method for the mirror-image machining support side of a large thin-film member on the basis of impedance control. According to the method, a support robot and a machining robot are used for carrying out mirror-image machining on the large thin-film member; the attitude and the position of a support mechanism are automatically adjusted according to the feedback information of a force sensor through adopting an impedance control strategy, so that mixed control on the position, the attitude and a force is realized; and the force sensor is used for measuring a support force signal, and feeding back the change conditions of a support force and a surface normal vector in real time, A/D conversion is carried out by virtue of a data acquisition card and then the data is transmitted to an industrial personal computer, the measured support force signal is processed, and correction is carried out on the original track, so that accurate and stable control on the support force is realized. The method is simple to realize, the support force is low in overshoot, and the method is suitable for the mirror-image machining for the large thin-film member and capable of increasing the machining efficiency and improving the machining accuracy.