There’re two bars outside the entrance of Minjiang river 0.2 million DWT entrance channel,i.e.the inner bar and the outer bar.It is a representative river estuary at which sand mainly comes from the land.This paper probes into two issues:1) To carry out an analysis on the formation causes of the outer bar according to the topography,flow & river type and based on the real-measured data concerning hydrology and sediment,and get the conclusion that the circulating current in the river bend is the main formation cause of the outer bar.2) Based on the analysis of the real-measured topographic map of the river estuary from 1886 to 1986,and considering the siltation process at the outer bar area,to propose the concept of “current having the characteristics of automatic transmission of resistance”,and explain theoretically the mechanism of diversion and bifurcation of the main current and put forward the condition of diverting.