Abstract: The flood control decision support system for the midstream of Fenhe River (FCDSSMFR) is a artificial intelligence interaction network between user and computer,which is based on the theories of management science,operations research,control theory and social science,solves problems in terms of computer simulation and information techniques and has assisting functions to the flood control decision makers. FCDSSMFR consists of the following eight sub-systems:Graphical information sub-system,historical storm flood simulation sub-system, storm water forecasting sub-system,real time flood forecasting and regulating sub-system,artificial storm water simulation sub-system,artificial intelligence sub-system,database management sub-system and sensitive help. The flood control expert system for the midstream of Fenhe (FESOF) is the artificial intelligence (AI) analysis part of FCDSSMFR,which is the first practical model of expert system for flood control field in China. This paper discusses the main ideas of the designing of FESOF's system softwares and building of flood control expert knowledge bases, and describes FESOF's execution and expansion.