Based on the process features of coagulation_flocculation,a new definition is given to the concept of coagulation_flocculation,with discussions made on the characteristics of the two stages of coagulation_flocculation process,coagulation stage and flocculation stage,as well as on the influencing factors. Issues to be noted during the coagulation_flocculation process in actual production and suitable technical indexes are pointed out through theoretical analyses and taking into consideration the practical operation.