The invention discloses a method for preparing a core-shell TiO2/SiO2 nano composite grain, using titanium tetrachloride, silicon titanium tetrachloride, hydrogen gas and air as raw materials, putting them into various links of a premixing combustion reactor for hydrolyzation reaction at 1200-2500 deg.C. And the produced grain is 20-80 nm sized, amorphous SiO2 relatively uniformly coats the crystalline TiO2 grain, where the coating is 2-30 nm thick, and size of TiO2 core and thickness of coating SiO2 can be regulated by variation of process parameters. And the process device is simple, convenient to operate, and stable to run, and able to continuously prepare the grain.