A preparation method of a SnO2-TiO2 composite nano photocatalyst comprises operation steps as follows: (1), mixing tetrabutyl titanate, absolute ethyl alcohol and glacial acetic acid, adding dilute nitric acid to regulate pH, stirring and adding water; (2) adding a SnCl4*5H2O solution into the mixed solution in a molar ratio of titanium to tin to be 3:1, stirring, and aging for 22-26 h; (3) placing the aged material into an autoclave for treatment for 22-25 h at the temperature of 175-185 DEG C, and cooling the material in the autoclave to the room temperature; and (4) performing centrifugal separation on a product, then drying the product at 95-110 DEG C and roasting the product for 2-6 h to obtain a finished product. The SnO2-TiO2 composite nano photocatalyst prepared with the method has higher purity and activity, good dispersibility, high thermostability and good repeatability; the method does not require addition of a surfactant and the like, so that the cost is lower, and the production cost is reduced greatly; and meanwhile, the production time is shortened to 2-3 d, and further, the operation is simple and safe.