All-inorganic perovskite CsPbX₃ (X = Cl, Br and I) quantum dots (QDs) have been wildly utilized in optoelectronic devices due to their tunable photoluminescence, high photoluminescence quantum yield (PLQY), and narrow-line width photoluminescence. However, the blue luminescence PLQY of CsPbX₃ perovskite quantum dots is still lower than their red and green luminescence counterparts (PLQYs nearly 100%). Here in this work, we present a handy strategy to synthesise the ultra-small blue luminescence Tin-doped CsPbBr₃ perovskite QDs by supersaturated recrystallization synthetic approach at room temperature, and the particle size of as-prepared QDs is lower than 4 nm. The crystal structure and optical property of Tin doped CsPbBr₃ QDs are characterized by XRD, TEM, ultraviolet-visible spectrophotometer, and fluorescence spectrophotometer. The results show that the particle size of as-prepared QDs is slightly shrunk from 3.33 nm (SnBr₂ 0.03 mmol) to 2.23 nm (SnBr₂ 0.06 mmol) as the SnBr₂ adding quantity increases, but there is no obvious change in the lattice spacing of doped QDs. The partial substitution of Pb for Tin leads the optical spectra to blue-shift from 490 nm (SnBr₂ 0.03 mmol) to 472 nm (SnBr₂ 0.06 mmol). The highest PLQY and the strongest XRD diffraction of ultra-small Tin doped CsPbBr₃blue luminescence QDs are obtained by adding SnBr₂ 0.05 mmol, and the blue luminescence peak is located at 472 nm with the PLQY of 53.4%. There is no any change in PL peak of Tin doped CsPbBr₃ QDs (SnBr₂ 0.05 mmol) by storing it under the ambient atmosphere for 15 days, and the PLQY of Sn²⁺ doped QDs is still 80% of the initial after 15 days. It is concluded that the crystallization and optical property can be effectively improved in Tin doped CsPbBr₃ QDs by partially replacing appropriate quantity of Pb by Tin.