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Component engineering and quantum confinement engineering approaches are commonly used to prepare blue perovskite quantum dots (PQDs). From a component engineering perspective, the hybrid halide (Br and Cl) perovskites can adequately perform bandgaptailoring and the as-obtained spectra could cover the entire blue light range. However, the mixed halide PQDs are prone to lattice mismatch and phase separation. In addition, the low formation energy of Cl vacancies in perovskites tends to form Cl vacancies within the band gap, leading to deeper defect energy levels and significantly nonradiative recombination. Small size PQDs with single halide (Br), such as CsPbBr3 with a size of ≈ 4 nm, exhibit pure blue emission due to quantum confinement effects, which can avoid the phase separation and the issue of Cl vacancies. However, current methods are difficult to form small quantum dots of uniform size, which results in a very wide linewidth. In this paper, a strategy is designed to accurately and controllably reduce the size of CsPbBr3 PQDs. With the post-treatment of the CsPbBr3 PQDs, the emitting color gradually changed from green to blue, and the QDs sizes are reduced from 12 nm to 3 nm. Finally, the PQDs exhibit the stable spectra with a range of 520 nm to 450 nm and highly reproducible PLQY of 60%. Correspondingly, the full width at half maximum （FWHM） is reduced from 27 nm to 15 nm. This strategy provides a new idea for the synthesis of blue-emitting PQDs and provides a research basis for further realization of efficient and stable PQDs and subsequent devices. [ABSTRACT FROM AUTHOR]