Considering the situation that the traditional shearography requires that the surface of the measured object must be sufficiently rough, thus it is hard to measure specular objects directly, an improved shearography system was proposed. The speckle generated by the laser passing through the frosted glass was reflected by the specular of the measured object to achieve the measurement. And we adopted double imaging Mach-Zehnder digital shearography to adjust the shear and carrier frequency independently and to expand the effective measurement region. In the defect detection experiments, the 8 k mirror panel was loaded by the thimble and the manufactured mirror material was loaded by heat loading. The experimental results revels that the distribution of defects detected by proposed method is basically consistent with that of manufactured defects and loading methods.