All-inorganic CsPbBr₃ single crystals have emerged as promising candidates for high-energy radiation detectors owing to their exceptional X-ray absorption capabilities, charge transport properties, and high stability. This study uses first-principles calculations to analyze the structure, band structure, and density of states of different CsPbBr₃ phases, demonstrating that the orthorhombic phase of CsPbBr₃ single crystals is more suitable for X-ray detector fabrication. A low-temperature solvent evaporation-induced crystallization method was used to fabricate single crystals to prevent defect introduction during phase transitions. After the introduction of NH₄SCN additives, the crystalline quality of the CsPbBr₃–NH₄SCN single crystals was significantly enhanced, with the μτ value and bulk resistivity increasing to 1.3 × 10⁻² cm²·V⁻¹ and 6.8 × 10¹⁰ Ω·cm, respectively. The ultimately fabricated CsPbBr₃–NH₄SCN single-crystal X-ray detector achieved a sensitivity level of 2,360.1 μC·Gy_air⁻¹·cm⁻² and a detection limit as low as 424.1 nGy_air·s⁻¹ under an electric field of 20 V·mm⁻¹. Overall, the experimental results provide new insights into the future fabrication of CsPbBr₃ single-crystal X-ray detectors.