The invention provides a preparation method of Barium Titanate nano-powder, which comprises the following steps that: soluble barium salt is added into H2O2 solution to be mixed continuously, the pH value of the solution is adjusted with ammonia water, and the mixed solution is held statically, filtered, washed and vacuum-dried to obtain a precursor BaO2. H2O2 powder; then the BaO2.H2O2 and H2TiO3 are mixed and ground, KOH and KNO3 are added to be used as a flux, then absolute ethyl alcohol is added for grinding, the ultrasonic treatment on the mixed ground powder is undertaken, and after being dried and removed ethanol, the mixture is dissolved with deionized water, centrifugally separated and washed with deionized water after being roasted and cooled; and finally the sample is put into thin HNO3 solution to be immersed so as to remove the foreign matters, and the sample is centrifugally separated, washed and dried to obtain the barium titanate powder with the granularity of about 15 to 40 nm. The barium titanate powder prepared with the method has the advantages of small power granularity, uniform particles, low roasting temperature, simple production process, safe production process, difficult accumulation of the product particles, high purity of the product, high production efficiency and low implementation cost, and can be widely used for the preparation of different inorganic nano functional materials.