The research progresses of application and mechanism of poly-dimethyldiallylammonium chloride(PDMDAAC) in raw-water treatment was summarized. After the introduction of the structure and properties of PDMDAAC, the development of application technology, the mechanism of enhanced coagulation and the security of PDMDAAC in potable water production were reviewed. The advantages of the technology of using composite coagulants containing PDMDAAC in treatment of tiny polluted raw water for potable water production were emphasized. It was indicated that the research of application technology combined with mechanism was one of the development directions in the future, for improving the performance of composite coagulant in the treatment of raw water with different quality.