The invention discloses a method for preparing mixed crystal nanometer titanium dioxide suspension from double precursors, which comprises the following steps of: (1) preparing TiCl4 solution and TiCl3 solution which have the titanium ion concentration of between 0.5 and 1.0 mol/L; (2) adding inorganic alkali solution into deionized water, adding mixed solution of the TiCl4 solution and the TiCl3solution until the pH value of the solution is between 1.0 and 2.5, crystallizing at the constant temperature of between 50 and 100 DEG C for 2 to 10 hours to obtain a white precipitate product, and filtering and washing to obtain a nanometer titanium dioxide wet sample; and (3) diluting the nanometer titanium dioxide wet sample by adding the deionized water, and performing ultrasonic dispersion to obtain the mixed crystal nanometer titanium dioxide suspension. The method is simple, the equipment requirement is low, the photocatalytic capacity of titanium dioxide generated by reaction is high, and the prepared suspension is stable. In addition, the suspension is prepared by using the wet sample directly, so that agglomeration can be reduced and prevented; and the suspension is diluted by adding the water and can be dispersed uniformly without ultrasonic.