The effect of limestone powder on rheological properties of cement paste with the same fluidity was investigated. In order to study its mechanism, Zeta potentiometer and optical microscope were employed to test the Zeta potential and microstructure of paste, respectively. Also the minimum water demand method was used to analyze wet packing density of particles in paste and water film thickness on the surface of particles. The results show that for paste with the same fluidity, yield stress is almost the same, but viscosity decreases gradually with the decrease of limestone powder fineness and increase content of limestone powder. The Zeta potential of limestone powder in aqueous solution is significantly higher than that of cement, which can significantly reduce the flocculation structure in cement paste and increase the packing density. The increased packing density indicates that more free water is released, thereby the thickness of the water film on the surface of the particles is increased and further the interaction force between particles is reduced, which leads to a significant reduction in viscosity of cement paste.