Abstract: The confluence of water flow that occurs in many hydraulic systems is usually very complicated in flow structure, and an in-depth examination of its behaviors is practically significant to the study of the confluences of a river main channel and the tributaries. This study adopts the PIV technology to overcome the shortcomings of single-point velocity measurements, observes the asymmetrical 3D flow structures of an open-channel confluence at different discharge ratios, and analyzes their variations with the discharge ratio. Results show that with an increasing discharge ratio, water surface slope decreases upstream of the confluence, while it increases over the junction area and in its downstream. And for the depth average velocity under an increasing discharge ratio, it is increased upstream of the confluence responding to the increasing backwater effect and over the junction area, but decreased in the flow separation zone. The PIV technology can better reveal the flow structure of a flow confluence and is applicable to the experimental study of the flows in open-channel confluences.