The conditions of coagulation process and of tiny-polluted raw water in China were taken as a background, the preliminary degradation of Poly(dimethyldiallylammonium) chloride（PDMDAAC）reacting with mono-chloramine, finally to form disinfection by product NDMA was taken as the research object, the reaction kinetics of mono-chloramine and PDMDAAC were studied by means of initial reactant concentration and isolation independently within a range of temperature 15~35℃in this paper. The results showed that the general reaction order was conformed to be more than 1.99, closed to 2; the reaction order for mono-chloramine and PDMDAAC were all more than 0.9971, and coefficient of correlation R2 was more than 0.9971. The reaction rate constant was 1.23×10-5L/(mol•h) at 25℃.And the reactive activation energy Ea was 44.22kJ/mol, approximately. Thus, the kinetics research on the starting stage of PDMDAAC reacting with mono-chloramine to form NDMA was completed.