In response to the issues such as poor accuracy and low efficiency of simulation results in the flood inundation algorithm based on DEM, a lateral inundation algorithm applicable to small and medium-sized rivers is proposed. The algorithm takes the river as the object and divides it into segments according to the intersection points of river confluences. Then, the inflection points of the river sections are extracted and lateral vertical detections are carried out at the equal division points between two adjacent inflection points in the same river section towards both sides of the river channel to obtain the inundation range of the river channel, thereby obtaining the flood inundation area. DEM data with resolutions of 30m, 90m and 1km in the Hengjiang River Basin were selected for simulation analysis. The results indicate that this method can effectively improve the ability of flood inundation analysis and realize the simulation of the flood inundation process in small and medium-sized rivers, providing rapid and accurate decision-making basis and technical support for the prevention of mountain torrent disasters and risk assessment in small and medium-sized rivers.