In this paper, a three-layer liquid crystal polarization grating is designed and implemented, and its diffraction characteristics and coupling in optical waveguide are studied. In theory, the transverse period of 424 nm is selected, and the corresponding three longitudinal periods is calculated according to the specific working wavelength of 450 nm, 532 nm and 633 nm to achieve the superposition of the thickness direction under the condition of Bragg diffraction. Secondly, the finite element method is used to simulate the device. Finally, the relationship between the incident angle and the diffraction angle coupled in the waveguide is simulated according to the red, green and blue light dispersion relationship. The liquid crystal polarization grating device is fabricated and its system performance is characterized. A preliminary optical waveguide coupling experiment is given to demonstrate its coupling effect. The results show that the diffraction efficiency of the prepared three-layer liquid crystal polarization grating device can reach more than 80% at 450 nm, 532 nm and 633 nm, and the angular bandwidth of the corresponding wavelength is 12°, 8° and 6°, respectively. The field of view of the device coupled in the waveguide can reach 27.81° by simulation, which is a good optical waveguide coupler.