The Xiadou bottle-neck is the most important control section of the Wulongjiang River. Its resistance characteristics and backwater effect on the flood level and flooding capacity of the Wulongjiang River are still unclear. A two-dimensional mathematical model was used to calculate the water surface profile, water surface slope and velocity distribution characteristics of the Wulongjiang River under different flood computational schemes, and the critical flow rate, the height and reach of backwater of Xiadou bottle-neck were analyzed. The results showed that the Xiadou bottle-neck has the maximum slope of water surface of the Wulongjiang River. When the flow rate of the Xiadou bottle-neck exceeds 19320 m3/s, the maximum average velocity of Wulongjiang River is located in the Xiadou bottle-neck, and its resistance characteristic and backwater effect are closely related to flow. It is preliminarily determined that the critical flow rate of the Xiadou bottle-neck is 19320 m3/s. When the flow rate of Xiadou is 35037 m3/s（P=0.5%）, 32333 m3/s（P=1%）, 29435 m3/s（P=2%）, the height of the backwater is 0.13 m,0.10 m,0.07 m, respectively. And the reach of the backwater located up to 1.99 km,1.88 km,1.69 km upstream of Xiadou bottle-neck.