It is common that the sewage is discharged into rivers through a municipal drainage network. The hydraulic properties of the confluence between a lateral drainage pipe and a rectangular open channel are investigated using numerical simulation. The simulation results show that the recirculation zone, the separation area, and the secondary flow are highly influenced by the tailgate height and flow rate ratios. Both the recirculation zone and the separation zone enlarge with the increasing flow rate ratios. The recirculation zone also enlarges with the tailgate height within a certain range. It is worth noting that as the tailgate height increases, two vortices rotating in opposite directions at the same level initially merge into a counter-clockwise rotating vortex, and then one vortex splits into three. Ultimately, one of them dissipates and there are two vortices left on a vertical line. The tailgate can reduce the vertical velocity gradient and the streamwise velocity to moderate the erosion of the channel bed caused by the pipe flow inlet.