The dam-break flood is always disastrous. To study the effects of branch dam-break flood on flood process, a 2-D hydrodynamic model integrating HLLC approximate Riemann solver was used to simulate the effects of dam-break flood in Qingshuigou Reservoir on flood process in Dali River. The simulation results showed that the calculated water level agreed well with the measured flood mark, confirming that the model is able simulate the evolution of dam-break flood in a reliable way. The dam-break of Qingshuigou Reservoir has obvious effects on both the upstream and downstream of Dali River. A backwater phenomenon and discharge tendency of decreasing-increasing-decreasing to stable were observed in the upstream reach of the Dali River. Regarding the flood discharge in the downstream, it increased firstly and followed by a descending pattern then became stable, instead of a simple superposition. This research can help guide the prevention the flood in the main stream caused by the dam-break floods in branch channels.