Abstract: Some solid material will deposit in the conjunction area when debris flow is in conjunction with the main river. The transport discharge,deposit discharge,transport ratio and deposit ratio are defined.Based on the momentum theory,we study not only the heading up and sediment deposit principle within the conjunction area but also the varying rule of discharge,density and moment transportation relation within the conjunction area after the confluence. We also deduce the equation of Froude number,which can reflect the hydraulic relation and confluence characteristic in the confluence area after conjunction of main river and branch river.It is verified with the experiment data. If the parameters are used correctly,the calculation value will be very close to the one got through observation.