Abstract: In this paper the flow properties and influence on the joint at downstream are studied.First,by analyzing on the development of boundary layer in the shallow supercritical flow and aeration properties,we find the character of energy dissipation that the resistance loss of shallow flow in the wide-steep channel is more than that of the flow in the general steep channel, and get its range value of the velocity coefficient. Then,according to the different flow and boundary condition,we analyze the flow momentum distribution in wide-steep channel,and find that when the ratio B/H<40 and the discharge is larger,the flow momentum lateral distribution in the end of steep channel shows a saddle profile which has stronger stability and anti disturbance;but when the ratio B/H >40 and the discharge is smaller,the momentum lateral distribution shows a elliptical profile which has bad stability and can't restrain the outside disturbance. Based on analyzing the we attains that the instability of shallow flow in the end of steep channel originates from lack of stronger both wings control in momentum lateral distribution, which brings about accumulation and increase progressively of minor disturbance and leads to instability of the end hydraulic jump and insufficiency of the energy dissipation. In order to improve stability of join flow at downstream,a measure adjusting of momentum lateral distribution is put forward in this paper.