The one-and two-dimensional hydrodynamic coupling numerical simulation models of flood control in dike burst caused mountain flood ditch were established, and the risk of flood inundation was analyzed and evaluated. The special boundaries such as road and irrigation canal were generalized as a wide crest weir and treated linearly. The unstructured grid encryption and dissection technology were used to couple the special boundary, so that the breaking embankment mountain flood coupling model could accurately reflect the real terrain of the calculation area. At the same time, the wet and dry depth theory was used to optimize the model calculation efficiency. The model was applied to calculate and simulate the break dike mountain flood routing once in 100 years occurred in the city section of Dawukou ditch in Ningxia Province. The evolution of ditch flood in the submerged area was simulated, and the distribution characteristics and impacts of flood inundation was analyzed. The research results could provide technical support for flash flood risk assessment and flood control planning of mountain ditches.