Interaction between cement and a kind of sulfate compatible polycarboxylate type admixture (PCA) and action mechanism of the novel water reducer were investigated. Experimental results indicate that adsorption amount of PCA on surfaces of cement particles is lower than that of polynaphthalene sulfonate condensate water reducer (PNS) because of the formation of more ettringite and calcium monosulfoaluminate, and calcium monocarboaluminate when CaCO3 presents in hydration system, even at initial period of hydration. Results from XRD analysis, derivative thermogravimetric (DTG) analysis and chemical analysis all verify the benificial effect of sulfate compatible PCA on initial and early age hydration of C3A. An establishment of electrostatic repulsion by grafting SO3- in molecule structure of PCA may result in a more negative zeta potential in the dispersion system. The improved dispersibility and fluidity retention are the result of a combination action mechanism of electrostatic repulsion cooperating with steric hindrance, and the acceleration effect of sulfate compatible PCA on initial and early age formation of ettringite and AFm phase. A combined high range water reducer mainly consisting of sulfate compatible PCA and PNS was made up based on the experimental and theoretical investigation results.