To realize efficient resource utilization of coal gangue, the influences of modification treatments on performance of coal gangue mortar using coal gangue fine aggregate were investigated. Coal gangue fine aggregate was modified through fly ash-cement slurry encapsulation modification, CO₂ mineralization combined with fly ash-cement slurry encapsulation modification, and epoxy resin encapsulation modification methods. The aim was to study the influences of these coal gangue fine aggregate modifications on the working performance, mechanical properties, and microstructure of coal gangue mortar to elucidate their mechanisms. The results demonstrate that compared to unmodified coal gangue mortar, the modified coal gangue mortar exhibits improved water absorption, mechanical properties, and microstructure. Among these modifications, epoxy resin encapsulation modification proves most effective with a 28.8% increase in compressive strength and a 32.4% increase in modulus of elasticity, while reducing water absorption by 25.3%. This can be attributed to the enhanced overall strength and reduced porosity of aggregates due to epoxy resin hardening, as well as improvements in interfacial transition zones, leading to enhanced mechanical properties.