Ascertaining the coagulative control index and clarifying the effect of eddy motion of flow theory are worth to pay attention in coagulation dynamics.How to select rational G and Gt was discussed under the optimum hydraulic condition,and on this basis,the eddy motion of flow at the corner of reciprocating baffled flocculation tank was studied.It can be seen that the rational coagulative control index is favorable to form even more and smaller infinitesimal eddy, and the effective collision is enhanced,as a result coagulative effect is improved apparently.