Engineering of drainage and water log prevention is an important measure to prevent and mitigate the flood disasters. The scale of the engineering is mainly restricted by rain flood variation within the project life period and the level of economic and social development. This paper discusses the characteristics of the changes of city rainwater, the causes of flood disaster and disaster prevention and reduction principle of different types of engineering measures. Additionally, it also explains the concept of disaster probability and design standard and presents the hydrological calculation task about planning and designing of engineering of drainage and water log prevention in urban areas. By doing it so, it discusses the rainstorm intensity formula, design rainstorm hydrograph and other problems, which are involved in city design storm. Finally, it is described the principle and method of confirming design flood from design storm.