To improve the forecast period of real-time urban flooding warning, a real time warning simulation method based on radar technology was proposed. The base reflectivity of feature time was predicted and the optimized Z-R relationship was used to compute rainfall. Then the rainfall in the future was obtained and taken as an input in the hydraulic model. By real time simulation, the urban flooding in the feature was obtained and the urban flooding real time warning was realized. The result shows that urban flooding distribution can be obtained through the urban flooding real time warning based on radar technique, and more than 50 minutes’ forecast period can be saved. Compared with rain gage-based real time simulation, the length of forecast period can be reduced by 5-25 minutes, thus more time was provided for flood prevention department. The rainfall spatial distribution was taken as input in this method. Compared with rain gage input, rainfall spatial distribution was taken into full consideration.