A method of preparing pigment-grade TiO2 by decomposing titanium slag with mixed acid comprises the steps of: 1) mixing the mixed acid including sulfuric acid and hydrochloric acid with the titanium slag to perform acidolysis, and filtering the product to obtain crude titanium liquid and residue; 2) regulating concentrations of titanium, acid and the impurities in the crude titanium liquid so as to easily perform hydrolysis to produce refined titanium liquid; 3) hydrolyzing and filtering the refined titanium liquid in the step 2) and removing impurities to obtain refined metatitanic acid; 4) treating the refined metatitanic acid in the step 3) with salt, and calcining the metatitanic acid to prepare the pigment-grade anatase/rutile TiO2. The method is high in resource utilization rate, iseasy to carry out industrially and is environment-friendly.