A mathematical model on the preparation process of polysilicic acid (PS) was established to joint the bench-scale study with industrialization.Poly-silicic-ferric (PSF) coagulant was prepared using PS as semi-manufactured goods.PS was prepared using water glass and H2SO4 solution,and mathematical modeling concerning the relationship between gelation time (t) and such factors as pH value,initial w (SiO2) and temperature was presented.The influence of pH value on the species of PS with certain polymerization degree was explored.PSF coagulant was prepared using PS,FeSO4·7H2O and NaClO3 by co-polymerization.Both coagulation performance in treating lower temperature and lower turbidity water by PSF and characteristics of PSF were investigated compared with those of polyferric sulfate (PFS) and poly ferric aluminum (PFA).Results show that some PS is depolymerized after decreasing pH value when PS reaches a certain degree of polymerization,but the species after depolymerization differs from that at the same original pH value.PSF has higher coagulation efficiency than PFS and PFA.The mathematical modeling on the preparation of PS is the basic condition for preparing high quality PSF in industrial scale.The particular characteristics of PSF are just the essential reason for its superior coagulation performance.In the coagulation mechanism of treating low temperature and low turbidity water by PSF,charge-neutralization/destabilization is the precondition and the bridging is the necessary condition.