The low carbonization of cement has been a research hotspot at home and abroadand in recent years, using active mineral admixture to replace cement is an effective method to reduce CO2 emission. In order to verify the feasibility of activated coal gangue as cement mineral admixture, the effect of activated coal gangue on rheological properties, mechanical properties, hydration products and hydration degree of cement were studied, and the effects of water-binder ratio, age and activated gangue content on compressive strength and flexural strength of cement mortar specimens were revealed. XRD, SEM and TG/DTG were used to characterize the effect of activated coal gangue on hydration products and microstructure of cement. The results show that the rheological properties of activated coal gangue cement are more sensitive to the change of water-binder ratio. The incorporation of activated coal gangue into cement can effectively reduce the early hydration rate of cement. The activated coal gangue contains a lot of activated SiO2 and Al2O3, and has strong rehydration activity. The secondary hydration products calcium silicate hydrate and calcium aluminate hydrate gel can fill the pores of cement body and improve the strength of cement matrix. Compared with 30% (mass fraction) activated coal gangue sample, the 28 d flexural strength and compressive strength of 30% (mass fraction) activated coal gangue sample increases by 11.69% and 11.82%, respectively.