The present invention relates to the field of lithium battery material preparation technologies, particularly to a method for preparing lithium iron phosphate using the by-product ferrous sulfate from titanium dioxide. The method comprises the following steps: adjusting the pH of water by adding sulfuric acid, then dissolving by-product ferrous sulfate from titanium dioxide in the water, and adding iron powder for stirring and reaction; adding iron phosphate or lithium iron phosphate waste powder to the solution, heating and stirring the mixture, allowing the mixture to settle and cool, and filtering the cooled mixture to obtain a purified ferrous sulfate solution; and adding phosphoric acid and a lithium hydroxide solution in a co-current manner in an autoclave, and finally adding the purified ferrous sulfate solution, heating the mixture under stirring, then filtering, washing, and drying the mixture to obtain lithium iron phosphate powder. In the present invention, during the purification of the ferrous sulfate solution, iron phosphate or lithium iron phosphate waste powder is used as a precipitant. The resultant ferrous sulfate solution has a titanium content of less than 10 ppm, with no impurity elements other than iron, phosphorus, and lithium introduced. The ferrous sulfate solution is used as an iron source for preparing lithium-ion battery positive electrode materials, and the material exhibits excellent electrochemical performance, achieving effective utilization of titanium dioxide by-products.