Metal halide perovskites (MHPs) have demonstrated excellent performances in detection of X-rays and gamma-rays. Most studies focus on improving the sensitivity of single-pixel MHP detectors. However, little work pays attention to the dark current, which is crucial for the back-end circuit integration. Herein, the requirement of dark current is quantitatively evaluated as low as 10-9 A/cm2 for X-ray imagers integrated on pixel circuits. Moreover, through the semiconductor device analysis and simulation, we reveal that the main current compositions of thick perovskite X-ray detectors are the thermionic-emission current (JT) and the generation-recombination current (Jg-r). The typical observed failures of p–n junctions in thick detectors are caused by the high generation-recombination current due to the band mismatch and interface defects. This work provides a deep insight into the design of high sensitivity and low dark current perovskite X-ray detectors.