Abstract: Organic-inorganic hybrid perovskite single crystals, particularly MAPbBr₃, have shown significant potential in photodetection due to their excellent optoelectronic properties, which are closely related to their crystalline quality. In this study, the efficient crystallization regulator methylammonium chloride (MACl), widely used in thin-film systems, was extended to the preparation of MAPbBr₃ single crystals. Using a low-pressure-induced crystallization method, MACl additive at concentrations of 5 mol% to 40 mol% (relative to Pb) was introduced into the MAPbBr₃ precursor solution. The effects of MACl on the macroscopic morphology, crystalline quality, optoelectronic properties, and X-ray detection performance were systematically investigated. Characterization results indicate that Cl⁻ from MACl can partially substitute Br⁻ in the lattice, passivating defects such as halogen vacancies and uncoordinated Pb²⁺. Subsequently, MACl volatilizes in gaseous form, significantly reducing the defect density in the crystal. The optimal doping concentration was found to be 15 mol% MACl, which increased the average carrier lifetime from 6.90 ns to 183.47 ns, reduced the defect density to 8.97 × 10¹¹ cm⁻³, and enhanced resistivity to 1.88×10⁷ Ω·cm. An X-ray detector with a planar structure fabricated from the optimized crystal demonstrated a sensitivity of 115.05 μC Gy_air⁻¹ cm⁻² under a bias voltage of 1 V, with the lowest detectable dose rate (Lod) reduced to 48.81 μGy_air s⁻¹. These results represent a significant performance improvement compared to undoped samples. This study confirms the effectiveness of the MACl additive strategy in enhancing the quality of MAPbBr₃ single crystals and the performance of X-ray detectors, providing valuable insights for the development of high-performance perovskite-based X-ray detectors.