Abstract Metal halide perovskites have been widely used in x-ray detection due to their outstanding optoelectronic properties. However, the dark current of perovskite x-ray detectors is not appreciably low for integration on thin-film transistors pixel circuits and thus limits their applications in X-ray imaging. Based on numerical models, we investigate the correlation between the dark current and defects of perovskite x-ray detectors. The deep-level defects are the major factor to induce dark current, which has a proportional relation to the defect density. Compared to deep-level defects, the dark current induced by shallow-level defects depends on both of defect energy level and defect density. At last, simulation results present a guidance to engineer defects with suitable values of density and energy level, which yields desirably low dark current. This work provides implications and theoretical guidance for the optimization of defects in halide perovskites, which is believed to assist the further development of x-ray detectors with a low dark current density.