The invention discloses a preparation method of polydimethyl diallyl 
ammonium chloride.  It is based on the industrial cationic 
monomer DMDAAC 
aqueous solution obtained by a high-yield one-step clean preparation process as a 
raw material, adding 
persulfate as an initiator and Na4EDTA complexing agent at one time, controlling the 
reaction temperature under stirring, and controlling the 
initiation rate according to gradient temperature rise  , initiate the 
polymerization reaction of 
monomer DMDAAC at a lower temperature, increase the 
degree of polymerization of the product PDMDAAC, improve the relative molecular weight, add an 
aqueous solution containing a small amount of initiator at a higher temperature in the later stage of the reaction to improve the 
monomer conversion rate in the product, and finally  By controlling the amount of 
dilution water added, the 
solid content of the product is adjusted.  The present invention is not only easy to obtain 
polymer products with higher relative molecular weight, but also has good economical and industrial bases for its preparation method; the 
initiation temperature is reduced, and the 
polymerization reaction is carried out under relatively mild conditions, which is easy to operate and low in production cost;  PDMDAAC products with different 
solid contents and different uses can be obtained.