The uniform growth of large-size optical grade diamond films has been one of the hot spots and difficulties in the field of microwave plasma chemical vapor deposition (MPCVD) diamond research, in which the structure and position of the deposition platform are crucial to the uniformity of diamond films and the long-term stability of thick film growth. Here, the effect of deposition platform height on the electric field uniformity, plasma state and temperature uniformity on the substrate surface was investigated by COMSOL simulation combined with experiments to optimize the process parameters for the uniform growth of optical grade diamond films. The 2-inch diamond film with thickness of 337 μm and inhomogeneity <11% was obtained at the optimal deposition platform height of 2 mm. The full width of the Raman half-peak from the center to the edge of the film is in the range of 3-4 cm^-1, and the maximum transmittance is 69%-70% in the visible light band and 70% in the infrared light at 10.6 μm. This indicates that thickness and quality of the diamond film are relatively uniform, thus the uniform deposition of 2-inch optical grade diamond film is achieved. Above results show that the deposition platform height has a great influence on the electric field distribution and plasma state on the substrate surface, and the electric field uniformity on the substrate surface is significantly improved with the increase of the deposition height, but the influence on the temperature uniformity is smaller.