Several polycarboxylic acid type water-reducing agents (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 the transformation from C2AH8 to Al (OH) 3 and C3AH6; for structure of the PCs, the bigger molecular weight of long-side-chain of PCs is advantage to restrain hydration of C3A. And it also is better to formation of six-party flake hydrated products. But the little molecular weight of long-side-chain of PCs tends to form the cube-shaped hydrated products.