To characterize the diffraction characteristics of the liquid crystal polarization grating when the beam is incident obliquely, a three-dimensional model modelling method of liquid crystal polarization grating oblique incident angle-driving voltage-diffraction efficiency was proposed. This method uses the Gibbs free energy equation to solve the director of the liquid crystal molecule, obtains the expression of the driving voltage and the tilt angle of the liquid crystal molecule, derives the relationship between the oblique incident angle and the phase retardation, and combines the extended Jones matrix to characterize different incident angles. The transmittance of the lower liquid crystal polarization grating, through the vector diffraction theory, establishes a three-dimensional model of oblique incident angle-driving voltage-diffraction efficiency. This model can not only quantitatively solve the diffraction efficiency of liquid crystal polarization gratings at different oblique incident angles, but also realize the calibration of the driving voltage when the diffraction efficiency is optimal. The effectiveness of the model was verified by simulation analysis and experiments. The results show that when the beam incident angle is tilted from 0° to 10°, the optimal driving voltage is reduced from 2.2 V to 2.0 V, and the diffraction efficiency of the liquid crystal polarization grating is reduced from 85% to 78%; when the beam incident angle is tilted from 0° to -10°, the optimal driving voltage is increased from 2.2 V to 2.4 V, and the diffraction efficiency of the liquid crystal polarization grating drops from 85% to 74%.