Flood is one of the most important natural disasters affect China greatly, especially for southeast cities. To help mitigate the loss brought by it, technologies such as simulation, visualization, decision support system, etc. could be utilized in the process of emergency management for city flood. Thus the Flood Simulation and Decision Support System (FSDSS) comes into being. It is an integrated decision support system that consists of sub-systems corresponding to most of the important parts of city flood emergency management, which include rainfall telemetry, flood forecasting, flood dispatching, flood visualization, resource deployment, disaster evaluation and so on. And it aims at providing information, tools, and solutions for the decision-makers during the whole process of city flood emergency management. This article describes the architecture, functions, implementation and construction of FSDSS in details and in the end an application case of this system in the city of Shenzhen is talked about.