Flood Risk Map is one of the important non-engineering measures for flood control and disaster reduction. On the basis of the flood risk analysis, and with the powerful spatial analysis and display functions of Arc GIS, the Flood Risk Map in flood detention area can be drawn automatically through the redevelopment of Arc GIS by Arc Engine. Firstly, a new layer can be created according to the boundary of the flood detention area. When simulating the inundate characteristics of specific flood regimes, the boundary of the flood detention area and the main water barriers within the flood detention area are used as the constraint boundaries. The computed grids layer can be created automatically for the 2-D hydraulic model. Meanwhile, after setting the time step in the flood simulation, some important hydrological elements such as water stage, velocity and flow direction can be computed for every grids at each time step; secondly, a series of fields which are corresponding to the hydrological elements can be created in the attribution table of the calculated grid layer, and importing the result of the flood simulation into the same attribution table, then, according to the values of the hydrological elements and the need of the visual effect, the fields in every calculation grids are colored automatically in classified levels by the corresponding color matching scenarios, thus obtaining the flood risk layer; finally, based on the classified results of the colored fields, do some operations such as merging and smoothing to the flood risk layer, and adding some other basic layers then forming a complete Flood Risk Map. As an example, the method was applied to mapping the Flood Risk Map for Mengwa flood detention area which is located on the left side of Huaihe River in Anhui province. It indicated that the procedure presented in this paper provides an alternative to draw the comprehensive Flood Risk Map by a quick and efficient way.