Abstract: An emergency assessment model is developed for the hazard extent of water pollution accidents in small and mediumsized rivers.The model development uses the concept of the hazard extent of water pollution accidents and the result of typical case studies.The rough set theory is applied to analyze the importance of indirect factors, eliminate the redundancy, calculate the significance of weight, and integrate the influence of both direct and indirect factors.The model is applied to a river in Chengdu city.Different water pollution scenarios are designed for the model smiulations.Results show that the model is capable of assessing the hazard extent of water pollution accidents in small and medium sized rivers.Thus, the model is able to provide the valuable information for decision-making during the emergency of water pollution accidents.