Due to the complex terrain, steep banks, zigzag coastline, and extremely irregular bed shapes of the mountain rivers, dam break can cause unpredictable flow patterns. The analysis of dam break in mountain rivers is aimed to protect mountain rivers from flood disasters and to provide a basis for flood control. According to the incompressible Navier-Stokes equations, a two-dimensional mathematical model for the dam break flow in mountain rivers was established. The model mesh was divided by the unstructured triangle mesh, and the dynamic boundary technology was used to process the wet and dry boundary. The obtained roughness ranged between 0.020 and 0.035 s/m1/3. The flood propagation characteristics under six dam-break conditions were investigated using the above model. The numerical results show that the mean velocity of each section was less than 6 m/s. The plane form of the river had a significant impact on the dam break flood evolution process. The contraction section and curved section of the river can effectively suppress the transmission of flood waves. The backflow of the tributary can greatly reduce the peak flow and the existence of the branch ditch can consume mainstream energy.