The invention relates to a method for preparing a neutral mixed crystal nano TiO2 hydrosol with high stability, belonging to the photocatalytic active nano titanium dioxide material preparation process technical field. The method comprises the main steps that: 1. a titanium compound is hydrolyzed by aqueous slkali and deposited, filtered and washed; 2. products obtained are added in an acid solution and peptized at the temperature of between 60 and 100 DEG C, and added with a certain amount of SiO2 sols used as a stabilizer and metal salt used as a doping agent to be continuously reacted for a period of time; 3. the mixture is added with alkaline liquor for adjusting the pH value to between 7 and 8; 4. the mixture is filtered and washed, and filter cakes are dissolved in a certain amount of water and redispersed, and finally the neutral mixed crystal nano TiO2 sol with high stability is obtained. The TiO2 compound hydrosol prepared by the invention is neutral and has good transparency and stability, and the crystal form ratio of anatase and rutile is controllable, and the spectral response range is wide; the TiO2 hydrosol prepared by the invention is convenient in coating operation; moreover, the hydrosol can utilize the solar energy to complete sewage disposal, air purification, antibiotic treatment and glass film coating treatment, etc.