Abstract: Based on the integrated spatial database, relation database and multimedia database, a geographical information system (GIS) of Nanjing City is set up. The system has the functions to analyze water and rain information, to forecast floods, and to assess losses casued by disastrous floods. It provides services for information management and in quiry in the aspects of social economy, engineering data, law, historiacal disasters and on-the-spot recording of floods. The system played an important role during the flood prevention period in 1998 in Nanjing city.