The influence of limestone powder on the hydration characteristics and rheological properties of cement was studied from two aspects of content and fineness by micro calorimeter and rotational viscometer. The influence of limestone powder on the system hydration was analyzed through the rate of hydration heat evolution and hydration heat， while the influence of limestone powder on the rheological properties was analyzed from two angles of close packing and particle volume fraction. The results show that limestone powder can promote the system hydration process， and the greater fineness of limestone powder results in the more obvious promoting effect. The addition of limestone powder leads to the decrease of cement content， so the second exothermic peak and total heat of the system decrease with the increase of limestone powder content. With the increase of limestone powder content or fineness， the volume fraction of solid particles increases gradually， the particle size distribution modulus decreases， and the particle size distribution curve of composite system is close to the ideal distribution curve gradually. The yield stress and plastic viscosity of composite systems decrease with the increase of limestone powder content， but increase with the increase of limestone powder fineness.