The rectangular plate structure under free boundary conditions is theoretically analyzed by elastic force in this paper. The geometry, physics and balance equations of the plate structure are established. By analyzing the pure bending case of the thin plate, the function expression of its deflection curve is obtained. The COMSOL software is used to simulate and analyze the PZT-based square plate under those conditions, and through parameter identification, the curve fitting of simulation curve and theory is carried out to verify the consistency of theory and simulation. The authenticity of the simulation data is verified by experimental test. On this basis, the deflection angle of the square thin plate can be solved, which can further realize the deflection of the beam by the thin plate in the field of laser communication, and achieve the function of accurately controlling the deflection angle of the beam.