Self-trapped excitons (STEs) emission from halide perovskites with strong exciton-phonon coupling has attracted considerable attention due to the widespread application in optoelectronic devices. Nevertheless, the in-depth understanding of the relationship between exciton-phonon coupling and luminescence intensity remains incomplete. Herein, a doping-enhanced exciton-phonon coupling effect is observed in Cs₃Cu₂I₅ nanocrystals (NCs), which leads to a remarkable increasement of their STEs emission efficiency. Mechanism study shows that the hetero-valent substitution of Cu⁺ with alkaline-earth metal ions (AE²⁺) causes a greater degree of Jahn–Teller distortion between the ground state and excited state structures of [Cu₂I₅]³⁻ clusters as evidenced by our spectral analysis and first-principles calculations. As a consequence, an X-ray detector based on these Cs₃Cu₂I₅:AE NCs delivers an X-ray imaging resolution of up to 10 lp·mm⁻¹ and a low detection limit of 0.37 μGy_air·s⁻¹, disclosing the potential of doping-enhanced exciton-phonon coupling effect in improving STEs-emission and practical application for X-ray imaging.