Study on risk assessment of urban flood is the key to ensure the operation safety of city. In existing researches, the dynamic impact factors were not taken into account, and the evaluation method based on heuristic algorithm depends on a large number of training samples. To solve the above deficiencies, dynamic risk assessment of urban flood based on improved genetic algorithm was proposed in this paper. In this assessment index system, disaster-inducing factors, hazard-inducing environment, hazard-affected body, disaster prevention and reduction ability are taken into considered. The dynamic assessment of urban flood risk for small sample is realized by using genetic algorithm improved by date augmentation, also the consistency and superiority of the method are verified by case study. The research results of this paper provide a new idea and a technical mean for the dynamic management of urban flood risk assessment, and have very important value of application and promotion.