In order to study the influence of narrow river reaches on flood evolution, the two-dimensional all-hydrodynamic model GAST was used for numerical simulation, with which the quantitative relationship between the narrowing of the river reach and the flood hydraulic characteristics of the ideal river and Bahe River was analyzed. The results show that the influence of the narrow section shape on water depth and flow rate in the flood evolution process in decreasing order is: V-shaped, U-shaped, trapezoidal and rectangular. When the shape of the sections are the same, the greater the narrowing of the section, the deeper the influence on water depth and velocity. The water level in the upstream of the narrow reach is higher than that in the downstream, but the velocity is on the opposite, so broadening the narrow reach can reduce the difference of water level and velocity between the upstream and downstream. Under the flood 8·19, the upstream water depth of the three working conditions (original narrowing degree is 64.4%, after dredging the narrowing degree of the three working conditions become 55.6%, 46.7% and 37.8% ) can be reduced to 0.669, 0.985 and 1.066 m, and the flow velocity difference can be reduced by 0.702, 1.592 and 2.550 m/s. When the flood volume increases, the water level and flow velocity of the river increase, so does the influence of narrow reach width change on water level and velocity. Under the three inflow conditions, the maximum water depth of the original reach is 2.177, 2.778 and 3.618 m, and the maximum water depth is reduced to 1.866, 2.367 and 3.175 m after increasing the narrowing degree to 37.8%. The analysis of hydraulic characteristics of the narrow river reach with different narrowing degrees can provide a reference for the flood control of narrow river reaches.