The confluence overflow facility with transverse weir is adopted in China to reduce the occurrence of overflow. Under normal conditions, most of the facilities do not overflow. The study on the hydraulic structure without overflow is helpful to improve the pollutant interception efficiency. In addition, the weir height equation for different intercepting pipe diameters was required. Full scale experiments and simulation studies were carried out, and the reliability of the simulation results was verified by experiments. The simulation describes the hydraulic structure and shows that the backflow area appears on the side of the chamber away from the interceptor and at the inlet of the pipe. The results show that when the ratio of weir height to pipe diameter is less than 1.2, the closure pipe is non pressure closure, when the ratio is from 1.2 to 1.75, it becomes half pressure, and when the ratio is greater than 1.75, it is converted to pressure. The increased rate of intercepting flow with the rise of weir height would change under the effect of different flow characteristics. Based on the experimental data distribution and the hydraulic characteristic, the weir height equation was deduced for the pipe diameter of 0.4 m. Other pipe diameter simulations showed that this equation was also applicable.