A low defect density in metal halide perovskite single crystals is critical to achieve high performance optoelectronic devices. Here we show the reduction of defect density in perovskite single crystals grown by a ligand-assisted solution process with 3-(decyldimethylammonio)-propane-sulfonate inner salt (DPSI) as an additive. DPSI ligands anchoring with lead ions on perovskite crystal surfaces not only suppress nucleation in solution, but also regulate the addition of proper ions to the growing surface, which greatly enhances the crystal quality. The grown CH NH PbI crystals show better crystallinity and a 23-fold smaller trap density of 7 × 10  cm than the optimized control crystals. The enhanced material properties result in significantly suppressed ion migration and superior X-ray detection sensitivity of CH NH PbI detectors of (2.6 ± 0.4) × 10  µC Gy air cm for 60 kVp X-ray and the lowest detectable dose rate reaches (5.0 ± 0.7) nGy s , which enables reduced radiation dose to patients in medical X-ray diagnostics.