In a reaction system using water as the main solvent, the stable TiO₂ nanocrystal sol was synthesized under the mild conditions of atmospheric pressure and low temperature (70^_oC) through the improved sol-gel method with controlling tetrabutyl titanate to fully hydrolyze and slowly polymerize. TiO₂/bentonite composite photocatalytic material was further prepared by the loading of TiO₂ nanocrystal sol on the surface of bentonite. XRD,SEM and BET were adopted to analyze the structure and morphology of as-prepared samples, and their photocatalytic activities were also investigated. The results show that the high water addition amount is conducive to the formation of TiO₂ nanocrystals. When the molar ratio of deionized water: etrabutyl titanate is higher than 167:1, the anatase TiO₂ nanocrystals appear in the sol system. TiO₂ nanocrystals are mainly loaded on the surface of bentonite rather than embed into the structure between bentonite layers. However, compared with a pure bentonite, the TiO₂ loading significantly increases the specific surface area of the material. When the molar ratio of deionized water: etrabutyl titanate is 192:1, the composite photocatalytic material shows the highest photocatalytic activity under UV light, and the degradation rate of methylene blue can reach to 93.8%.