In order to prepare large-thickness near-stoichiometric lithium tantalate (nSLT) crystals by diffusion method, the high temperature lithium-rich diffusion process of lithium tantalate crystals were studied from the aspects of the ratio and synthesis process of Li-rich polycrystalline powders, polarization process and diffusion conditions. The lithium content, poling fields and optical homogeneity of prepared crystals were characterized, and the effect of diffusion conditions on crystal composition was studied. The results show that the repeatable use of lithium-rich polycrystalline material with Li/Ta of 60/40 has no significant effect on the composition, domain inversion voltage and optical quality of the prepared crystal wafers. Based on the research, a series of nSLT wafers were prepared. The maximum thickness of the wafer is 3.2 mm, its composition reaches the stoichiometric ratio and the composition is uniform, the coercive field is about 152 V/mm, and the crystal optical quality meets practical requirements.