The invention discloses a preparation method of a mixed crystal type nanometer titanium dioxide. The method comprises the following steps of adding the deionized water in the metatitanic acid which is washed repeatedly for removing the sulfate radical therein to prepare a metatitanic acid slurry of which the mass percent is 10 to 12, adding titanium tetrachloride aqueous solution of which the mass percent is10 to 50 in the metatitanic acid slurry in the stirring process so as to obtain a precursor suspension, heating the precursor suspension to be 80 to 100 DEG C, preserving the temperature, reacting 60 to 600 minutes to finally obtain the mixed crystal type nanometer titanium dioxide suspension, regulating the pH of the mixed crystal type nanometer titanium dioxide suspension to be 6 to 8 with alkali, filtering, repeatedly washing and removing the impurity ions, drying and crushing the obtained filter cake to prepare the mixed crystal type nanometer titanium dioxide powder. The preparation method of the invention prepares the mixed crystal type nanometer titanium dioxide in a mild condition, which avoids the calcining step at high temperature, saves energy. The metatitanic acid and titanium tetrachloride are cheap and easily obtainable, the processes are short, and the operations are convenient, thereby being applicable to the large-scale industrialized production.