It is a hot research issue to utilize the computer and information technology to study flood disas-ter in recent years. Compared with cities

especially megacities

however small towns do not receive dueattention in flood reduction from a broad range. In this paper

the application of VR-GIS technology toanalysis and simulation of flood undulation in small towns is presented. A series of crucial technologicalproblems are solved

which consist of the construction of spatial database in small towns

three-dimension-al visualization

the determination of flood undulation area and the dynamic scenario of flood undulation.Based on three-dimensional visualization of the terrain and building in small towns

the flood undulationprocess can be analyzed and simulated statically and dynamically

so as to provide effective decision sup-port for prevention and reduction of the flood. The model and technology have been successfully imple-mented in the "GIS-Based Decision Support System for Reducing Disasters of Infrastructure in Smalltowns" sponsored by a national project.