Mn²⁺-doped metal halide perovskites present remarkable optical properties in optoelectronic applications, although the realization of high efficiency and stability is still a challenge. In this work, a series of highly efficient and stable orange-emitting Mn²⁺ alloyed Cs₄Cd_1-xMn_xBi₂Cl₁₂ single crystals are successfully synthesized via a hydrothermal reaction. Combined with the crystal structure and spectral characterization at 7 K, the site occupation of Mn²⁺ and defect emission are systematically discussed. Benefiting from the effective [BiCl₆]³⁻→[MnCl₆]⁴⁻ energy transfer and lattice distortion, these single crystals exhibit a maximum internal and external quantum yield of ≈97% and ≈65% at 35% heavy doping level. Interestingly, these Mn²⁺-alloyed single crystals exhibit remarkably waterproof stability, no decrease in emission intensity is observed after immersion in deionized water for 4 h. After soaking in deionized water for 100 days, the internal quantum yield can still maintain 44%, implying good chemical stability and moisture resistance due to the formation of protective BiOCl layer. This work provides new insights into the optimization mechanism for Mn²⁺ luminescence and overcoming the downside of their waterproofing in humid conditions.