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The double perovskite Cs2AgBiBr6 single crystal holds great potential for detecting applications because of its low minimum detectable dose rate and toxic‐free merit. Nevertheless, the disordered arrangement of Ag+/Bi3+ usually gives rise to unexpected structural distortion and thereafter heavily influences the photoelectric properties of the Cs2AgBiBr6 single crystal. Herein, phenylethylamine bromide is demonstrated to be capable of in situ regulation of the order–disorder phase transition in the Cs2AgBiBr6 single crystal. The improved ordering extent of alternatively arranged [AgX6]5− and [BiX6]3− octahedra is theoretically and experimentally proven to decrease the defect density and suppress self‐trapped exciton formation, and thereby tune the band gap and enhance the carrier mobility, which consequently promotes its application in an X‐ray detector. The performance of a corresponding detector based on PEA‐Cs2AgBiBr6 single crystal displays superior performances, e.g., longer carrier drift distance, higher photoconductive gain, and faster current response (13 vs 3190 µs). Prominently, the as‐fabricated PEA‐Cs2AgBiBr6 single‐crystal X‐ray detector has an extremely high sensitivity with a value of 288.8 µC Gyair−1 cm−2 under a bias of 50 V (22.7 V mm−1), which largely outperforms those of their counterparts with lower ordering structure. [ABSTRACT FROM AUTHOR]