The oxidation rate of ferrous sulfate is investigated for the preparation of polyferric sulfate(PFS) coagulant. It is proved that this reaction is zero order with respect to Fe 2+ , first order with respect to NO 2(g), and first order with respect to the interface area between gas phase and liquid phase. According to this mechanism, sectionalized reactor(SR) is used in place of traditional reactor(TR), and the liquid of reaction mixture is recycled by pump. As a result, not only the flow path of reaction liquid is prolonged, but also gas liquid contact area enlarged, and the reaction distinctly accelerated, compared with traditional reactor. The effects of parameters including temperature, acidity and others on the reaction rate are also discussed.