Abstract: Based on a large group of the flume experiments and the 3D Acoustic Doppler Velocimeter(ADV),we research the variation of the surface slope and the distribution characteristics of the average velocity around the 90° junction zone with different discharge ratios. And the result shows that the jacking action of the main and the tributary flow has some effect on the surface slope of the upstream of the entrance;the surface slope of entrance and the mixing area of the confluence along the down-stream increases with the rising of the discharge of the main stream,and decreases with the rising of the discharge ratio;the vertical distribution of the averaged velocity of different notes are doubly influenced by the discharge ratio and the flow pattern, to be exact,the average velocity of the upstream of the entrance decreases gradually with the increase of the discharge ratio, while the high and low velocity zone emerges obviously and the separation zone is formed along downstream of the entrance.