Organic modifier (PSFD) of poly-silicic-ferric (PSF) coagulant was prepared by adding dimethyldiallylammonium chloride (DMDAAC) as additives in PSF. The size distribution of PSFD using Photon Correlation Spectroscopy (PCS) method was characterized, in comparison with that of PSF. The results show that the increasing of PSFD product’s size was far larger than that of PSF at lower level. The size of PSFD hydrolysis products tended to change continuously and small size of materials disappeared after its hydrolysis, while the size of PSF particles had very uneven distribution and there always existed small size of hydrolysis products. The disappearance of size change against level between PSFD and PSF gives PSFD a higher bridging ability than PSF in real wastewater treatment process.