The polycarboxylate water reducers(PCs) with different molecular structure were synthesized. The effects of molecular structure on their performance in cement paste were investigated. The results show that the PCs with a medium density of PEO side chain and/or a moderate molecular weight and its distribution have a higher dispersibility and a stronger dispersion retention stability at the same time, and that those PCs with a shorter PEO side chain exhibit a stronger dispersion retention stability. The PCs with a higher molecular weight, a lower density and/or a shorter length of PEO side chain are more easy to adsorb on the surface of cement particles. The earlier induction period is shortened while the dormant period is prolonged during the process of cement hydration in the presence of PCs. The PCs with a shorter PEO side chain show a stronger retarding effct. This effect is not very obvious with the decreasing molecular weight.