Several polycarboxylate superplasticizers (PCs) with different side-chain structure have been synthesized. The effect of side-chain structure of PCs on hydration of C3A has been investigated by XRD, TG-DSC and SEM. The results show that PCs restrain hydration of C3A as a whole, but accelerate formation of Al(OH)3 and C3AH6; with increase molecular weight of long-side-chain, hydration of C3A is slowed, and the formation of Al (OH)3 and C2AH8 also become slowed. With increase proportion of long-side-chain, hydration of C3A and formation of C3AH6 are slowed, but existence of C2AH8 become obvious. The six-party flake hydrated products of C3AH6 are formed with a large number of grafted carboxyl.