A three-dimensional mathematical model that simulates the dam-break flood routing process that occurs during local cross-section reduction has been built to investigate the effects of local cross-section reduction on dam-break flow propagation. By calculating Reynolds-averaged Navier-Stokes equations basing on pressure implicit with splitting of operators(PISO), the model is built on the basis of the RNG turbulence model. The governing equations of dam-break flow are discretized with the finite volume method, the free surface flow is dealt with using the fluid volumetric function approach. The physical variables are arranged on a staggered grid, and then the values of dam-break flow with local cross-section reduction are simulatively calculated. The simulation results are in agreement with physical model test results. The formation and propagation process of negative waves in a dam-break flood were examined with the model. The formation and propagation laws of negative waves as well as the variation characteristics of the water level were obtained.