Although metal halide perovskites have advanced rapidly in photoelectrical detection, studies on perovskite-based photon-counting detector remain scarce. This study reports an efficient perovskite photon-counting detector achieved through defect passivation of the perovskite photosensitive layer. Urea, a Lewis base, was introduced as an additive during the two-step fabrication of MAPbI₃ films, forming a complex (MAI·PbI₂·O=C(NH₂)₂, where MAI = methylammonium iodide) that slowed the reaction rate and promoted complete reaction between MAI and PbI₂. Compared to conventional perovskite films, the optimized MAPbI₃ film exhibits enhanced crystallinity, with enlarged grain size from 254.8 to 908.1 nm, and reduced defect density from 9.08 × 10¹⁵ to 3.89 × 10¹⁵ cm⁻³. The resulting MAPbI₃ photodiode detector demonstrates satisfactory performance, achieving a responsivity of 0.4 A/W and a sensitivity of 2.39 × 10¹³ Jones under 17.02 μW/cm² illumination. Furthermore, a photon-counting system integrated with the MAPbI₃ photodetector was able to detect low light levels down to 2.01 × 10³ photons per mm², with a photoresponse three times higher than that of silicon-based photon-counting detector.