Abstract: Several special mechanical properties, such as dilatancy and compressibility, of cemented paste backfill (CPB) are controlled by its internal microstructure and evolution. The mesoscopic structure changes of CPB during the development process were investigated. On the basis of the scanning electron microscopy (SEM) and mechanical test results of CPB, the particle size information of CPB was extracted, and a two-dimensional particle flow code (PFC) model of CPB was established to analyze the evolution rule of mesoscopic parameters during CPB development. The embedded FISH language in PFC was used to develop a program for establishing a PFC model on the basis of the SEM results. The mesoscopic parameters of CPB samples at different curing times, such as coordination number (C_n), contact force chain, and rose diagram, were obtained by recording and loading and used to analyze the intrinsic relationship between mesoscopic parameter variations and macroscopic mechanical response during CPB development. It is of considerable significance to establish the physical model of CPB using the PFC to reveal the mesoscopic structure of CPB.