Dispersion mechanism of polycarboxylate type admixture (PCA) in high volume fly ash (HVFA) concrete was studied by testing fluidity of cement paste, zeta potential of cement grains, the adsorbed amount of PCA on surfaces of cement grains and degree of hydration was quantified by loss on ignition measurements. Properties of HVFA concrete were experimentally validated, and microstructure of HVFA concrete was observed by SEM. Experimental results indicated that a combination of PCAs with long and short backbones may enhance the fluidity and fluidity retention of cement pastes. Fly ash has the ability to compensate electrostatic repulsion of PCA by promoting adsorption-dispersion of PCA to cement minerals and making zeta potential of cement grains more negative. HVFA concrete incorporating combined PCA has excellent workability and appropriate strength. Microstructure of HVFA self-compacting concrete is denser for the modified homogeneous of concrete and a higher degree of hydration of cement achieved in the presence of PCA and fly ash. So combined polycarboxylate based admixture is absolutely necessary for producing high volume fly ash concrete with better workability and durability.