Quaternary ammonium salt (QAS) biocide and the researches of its biocidal performance and mechanism were reviewed. Based on the development course of existing QAS seven generations products, they were classified into four types biocides, i.e. single-chain QAS, double-chain QAS, polymer QAS and mixed QAS, according to the structural characteristics. The biocidal performance and mechanism of QAS biocide were described by combining the structural characteristics of four types biocides and using bacterial and algae as the action objects. Six basic steps of biocidal process and four basic modes of biocidal action were concluded. Based on the statements above, it was especially emphasized that study on the key action site and lethal action mode of QAS molecule on its action object was the main points of mechanism research. Aiming at concentration of a concrete action object, the researches of obtaining the relationship between the concentration and action time of biocide were the important foundation of application. Finally, some research suggestions of the biocidal performance and mechanism of QAS biocide were put forward.