In the design of full-color waveguide holographic head-up display, the color is always uneven or the structure is over-complex. A novel double coupling grating structure is designed and optimized with embedded layers of different refractive indexes and thicknesses. Based on rigorous coupled-wave analysis (RCWA), the calculations show that the structure can achieve a more uniform color imaging in the primary field of view. It′s simplified and reliable for mass production. The double-grating is designed with central wavelength of 470 nm and 632 nm, respectively, to compensate the diffraction energy. After optimization, when the incident angle changes during the range from -11° to 11°, the diffraction efficiency (DE) of the RGB wavelengths is greater than 85% in the primary field of view and the maximum efficiency differs by less than 5%.