Enhanced luminescence in rare-earth-doped chalcogenide glass–ceramics is of great interest for the potential integrated optoelectronic devices. However, fundamental mechanism on the enhancement of luminescence upon crystallization remains largely unknown. We report the fabrication and characterization of wide transmission chalcogenide glass and glass–ceramics based on the 25GeS2·35Ga2S3·40CsCl:0.3Er glass composition, and discuss the mechanism of enhanced luminescence. By monitoring the 4I9/2–4I15/2 of Er3+ transition, up-conversion luminescence of 12 times higher was observed in glass–ceramics compared with that in base glass. Electron paramagnetic resonance (EPR) and Raman scattering spectroscopies were employed to obtain the information of selective environment of Er3+ ions and microstructural evolution with the crystallization progress. Both of them evidenced that the enhanced up-conversion luminescence was mainly related to the local environmental evolution from a mixed chlorine-sulfur coordination to a low phonon energy chlorine coordination in the residual glassy matrix of glass–ceramics.