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　µGyair·s−1,　disclosing　the　potential　of　doping-enhanced　exciton-phonon　coupling　effect　in　improving　STEs-emission　and　practical　application　for　X-ray　imaging.[Figure　not　available:　see　fulltext.]　©　2023,　Tsinghua　University　Press.