The invention provides a modified titanium dioxide applied to mortar and a preparation method thereof. The preparation method comprises the following steps that (1) pretreatment is carried out on industrial titanium dioxide raw materials; (2) pore-forming treatment is carried out on TiO2 obtained in the step (1); (3) the outer layer of a silicon dioxide inner core is coated with a layer of TiO2; (4) the outer layer of TiO2 obtained in the step (3) is coated with a layer of Cr2O3; and (5) the outer layer of the titanium dioxide obtained in the step (4) is coated with a layer of TiO2. Accordingto the modified titanium dioxide applied to the mortar and the preparation method thereof, the titanium dioxide finished product has the characteristics of good mechanical strength, high photocatalysis activity and the like, is applied to the mortar, and can effectively prevent the coating from being polluted.