An experimental investigation was carried out to evaluate the effect of limestone powder with equivalent replacement of cement on the rheological properties of cement paste at different dosage of superplasticizers based on Bingham model. The results indicate that the larger specific surface area and smaller apparent density of limestone powder than cement led to stronger adsorption effect on water and lower water powder ratio(by volume), which deteriorate the rheological properties of cement paste, while its good grain composition, favourable particle morphology and promotion effect on the adsorption of cement particles to superplasticizers ameliorate it. These factors of two aspects result in an obvious critical point of the dosage of superplasticizers.