A method for preparing nanometer titanium dioxide, characterized in that, it comprises the following steps: dissolving ilmenite powder using hydrochloric acid to obtain a raw ore solution; eliminating the iron element in the raw ore solution to obtain a final solution containing titanium ions; heating the final solution for hydrolysis to obtain a hydrolyzed product containing titanium dioxide; and calcining the obtained hydrolyzed product to obtain nanometer titanium dioxide; in the step, the temperature for dissolving ore is 60∼100oC; and the step comprises: crystallization of ferrous chloride: cooling the raw ore solution obtained in the step to get crystalline ferrous chloride tetrahydrate, and separating crystalline ferrous chloride tetrahydrate by filtration to give a first solution; oxygenation: adding oxidizing agent to the first solution to oxygenate ferrous chloride remained in the first solution to ferric chloride so as to give a second solution; extraction: performing solvent extraction to the second solution to give a stripping solution containing ferric ions and raffinate containing titanium ions; and removal of silicon: removing silicon from the raffinate to obtain the final solution; wherein the step comprises the following steps (2c 1) and (2c 2), and in the step, silicon in the raffinate containing titanium ions obtained in the step (2c 2) is removed; the nanometer titanium dioxide obtained in the step is anatase type titanium dioxide powder: (2c 1) first solvent extraction: performing 3∼5 stage continuous extraction to the second solution with the organic oil phase comprising amine-containing extractant to obtain a stripping solution containing ferric ions and raffinate containing titanium ions; and (2c 2) second solvent extraction: performing second extraction to the raffinate containing titanium ions obtained in the step (2c 1) with oil phase containing organic phosphorous extractant, so as to give raffinate containing titanium ions in oil phase and a stripping solution containing hydrochloric acid in water phase, wherein the extraction process is 3∼5 stage continuous extraction, the organic phosphorus extractant includes organic phosphorus compound or mixture thereof, which has a general formula of R 1 R 2 R 3 PO, wherein R 1, R 2 and R 3 is linear or branched alkyl, and the sum of carbon atoms in R 1, R 2 and R 3 is more than 12; and wherein in the step, silicon in the raffinate containing titanium ions obtained in the step (2c 2) is removed. dissolving ilmenite powder using hydrochloric acid to obtain a raw ore solution; eliminating the iron element in the raw ore solution to obtain a final solution containing titanium ions; heating the final solution for hydrolysis to obtain a hydrolyzed product containing titanium dioxide; and calcining the obtained hydrolyzed product to obtain nanometer titanium dioxide; crystallization of ferrous chloride: cooling the raw ore solution obtained in the step to get crystalline ferrous chloride tetrahydrate, and separating crystalline ferrous chloride tetrahydrate by filtration to give a first solution; oxygenation: adding oxidizing agent to the first solution to oxygenate ferrous chloride remained in the first solution to ferric chloride so as to give a second solution; extraction: performing solvent extraction to the second solution to give a stripping solution containing ferric ions and raffinate containing titanium ions; and removal of silicon: removing silicon from the raffinate to obtain the final solution; (2c 1) first solvent extraction: performing 3∼5 stage continuous extraction to the second solution with the organic oil phase comprising amine-containing extractant to obtain a stripping solution containing ferric ions and raffinate containing titanium ions; and (2c 2) second solvent extraction: performing second extraction to the raffinate containing titanium ions obtained in the step (2c 1) with oil phase containing organic phosphorous extractant, so as to give raffinate containing titanium ions in oil phase and a stripping solution containing hydrochloric acid in water phase, wherein the extraction process is 3∼5 stage continuous extraction, the organic phosphorus extractant includes organic phosphorus compound or mixture thereof, which has a general formula of R 1 R 2 R 3 PO, wherein R 1, R 2 and R 3 is linear or branched alkyl, and the sum of carbon atoms in R 1, R 2 and R 3 is more than 12; and