By using I2 as an oxidant and CH3CN as a reaction medium, few-layer Mg-deficient borophene nanosheets (FBN) with a stoichiometric ratio of Mg0.22B2 are prepared by oxidizing MgB2 in a mixture of CH3CN and HCl for 14 days under nitrogen protection and followed by ultrasonic delaminating in CH3CN for 2 h. The prepared FBN possess a two-dimensional flake morphology, and they show a clear interference fringe with a d-spacing of 0.251 nm corresponding to the (208) plane of rhombohedral boron. While maintaining the hexagonal boron networks of MgB2, the FBN have an average thickness of about 4.14 nm (four monolayer borophene) and a lateral dimension of 500 nm, and the maximum Mg deintercalation rate can reach 78%. The acidity of the reaction system plays an important role; the HCl reaction system not only facilitates the oxidation of MgB2 by I2, but also increases the deintercalation ratio of Mg atoms. Etching of the Mg atom layer with HCl, the negative charge decrease of the boron layer by I2 oxidation, and the Mg chelating effect from CH3COOH due to the hydrolysis of CH3CN in an HCl environment led to a high deintercalation rate of the Mg atom. Density functional theory (DFT) calculations further support the result that the maximum deintercalation rate of Mg atoms is about 78% while maintaining the hexagonal layer structure of boron. This research solves the problems of low Mg atom deintercalation rate and hexagonal boron structure destruction when using the precursor MgB2 to produce borophene nanosheets, which is of great significance for large-scale novel preparation and application of borophene nanosheets.