To reduce the diffraction efficiency of liquid crystal spatial light modulator, an optimization method of diffraction efficiency of liquid crystal spatial light modulator based on spline interpolation is proposed. According to Tyman-Green interference principle, the phase modulation system is built. Loading on the modulator with a grayscale map of step change, the phase modulation curve of liquid crystal spatial light modulator is drawn by calculating the movement of interference fringes. The cubic spline inverse interpolation method is used to correct the phase modulation curve and to realize the compensation of the phase modulation amount. The diffraction efficiency test system of liquid crystal spatial light modulator is set up, and the experimental verification of the proposed optimization method is carried out and compared with the stochastic gradient descent method. The results show that when the beam deflection angle is 1.56°, 0.78°, 0.39° and 0.19°, the diffraction efficiency of the proposed method can increase by 30% to 40%. Compared with the stochastic parallel gradient descent method, the diffraction efficiency increases by 2%-8%. This method can effectively suppress the gate lobe energy, improve the diffraction efficiency of the main lobe beam, and overcome the disadvantages of the stochastic parallel gradient descent method, such as a number of iterations, slow optimization speed, and easy to fall into local optimal.