Abstract: Previous three-dimensional numerical simulations of dam-break floods lacked detailed terrain modeling for complex underlying surface in urban regions. This paper presents a fine terrain modeling method based on TIN and NURBS. The method can be combined with an equivalent roughness method to distinguish the influences of flow resistance in different underlying surface conditions, so that it provides detailed underlying surface information for simulations of dam-break flood routing. To simulate the flood routing, a modified RNG k?ε model coupled with the VOF method was adopted and its accuracy was verified by model validation. We simulated dam-break flood cases for an urban reservoir with complex downstream inundation areas and analyzed the hydrological information including flood routing process, flow rate and water depth. The results show that the terrain modeling method reduces data storage and has good accuracy, and that the dam-break flood model simulates flood routing satisfactorily, thus reflecting the complex flow patterns and the details of the flows.