X‐ray detectors have broad applications in medicine and industry. Although flexible lead‐free perovskite films are competitive because of their lightweight and low toxicity, they are less efficient due to low charge transport. Herein, we report low‐toxicity, flexible X‐ray detectors based on p‐type doped MA3Bi2I9 (MA=methylammonium) perovskite‐filled membranes (PFMs). Strong coordination between dopant 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4‐TCNQ) and MA3Bi2I9 and the establishment of charge‐transfer complex (CPX) improved the conductivity by four times. The flexible X‐ray detector achieved a high sensitivity of 2065 μC Gyair−1 cm−2 and an ultra‐low detection limit of 2.71 nGyair s−1, which is among the highest values in all environmentally friendly flexible X‐ray detectors. Importantly, the PFMs retained excellent charge transport under mechanical stress. All of those make flexible MA3Bi2I9 membranes more competitive as next‐generation X‐ray detection. Molecular doping improves the conductivity of flexible lead‐free MA3Bi2I9 perovskite membranes by four times. Flexible X‐ray detector achieves a high sensitivity of 2065 μC Gyair−1 cm−2, an ultra‐low detection limit of 2.71 nGyair s−1, a high mechanical robustness, which demonstrate potential in next‐generation X‐ray detection.