The effect of limestone powder content and fineness on the compressive strength of mortar was studied, and the hydration kinetic parameters of cement paste were calculated based on hydration heat test and BNG model, the pore structure of mortar was analyzed by capillary water absorption test and mercury injection test, and the action mechanism of limestone powder on mortar compressive strength was discussed. The results show that the compressive strength of mortar decreases with the addition of limestone powder, and the decreasing degree increases with the increase of limestone powder content and the decrease of fineness. Limestone powder reduces the hydration exothermic rate, hydration heat and nucleation rate KN and growth rate KG of cement paste, thus reduces the hydration degree of cement paste and then increases the capillary water absorption, porosity and the most probable pore size of mortar, and finally leads to the decrease of compressive strength of mortar.