Large shape errors can be found from the stage of fine grinding to the beginning of polish, it is proved that it can′t satisfy the measuring requirements to test the mirrors with profilers or ordinary interferometer in that stage. To obtain the optical mirrors′ shape error based on fringe reflection, the method of high precision and large dynamic is proposed. The principle and system errors of the testing system were analyzed. To verify the feasible of the testing system, the model of testing on a reflecting mirror (the aperture is 100 mm) was built and simulated by ray tracing. The preliminary testing data on rotational symmetry spherical mirror is: PV is 0.523 μm, RMS is 0.086 μm, The feasibility of the testing system based on fringe reflection used in this period is verified.