A method of deformation measurement is introduced for a rotating plane reflective mirror with high speed. Since the inertia moment is added by mirror rotating, a bend deflection will appear on the body of plane reflective mirror, which will influence the quality of laser beam capture. With regards to this, strains are measured in the mirror body by strain gauges, and the signals are transferred for data acquisition through a slip ring. Based on the relationship between deformation and strain, the corresponding equivalent thickness of mirror body can be estimated by simulation, and the deformation of mirror body can be reconstructed by digital integration. Compared with the finite element calculation results, the correctness of the measurement is verified. Research experiment shows that the proposed method is successful in the result of measurement, and it gives new clue of the deformation estimation of rotating mirror.