With the consequences of urbanization trends and climate changes, the impacts and interactions of flood disasters have become significantly complicated. As a complex system engineering, flood disaster risk assessment takes on very important meaning in the theory and practice of flood control. Flood disaster risk is the consequence of the interactions between different factors in a complicated flood disaster system. In this paper, building risk dynamic assessment conceptual method for rainstorm flood had been proposed using agent based modeling (ABM) according to the characteristics of flood disaster complex system. Rainstorm flood building risk dynamic assessment multi agents systems (RFBRDAMAS) had been constructed. The interior structures, procedures, mechanism and the rules of different agents in RFBRDAMAS had been designed. The communication coordinator of different agents was focused. The proposed method can be used to dynamic assessment of building risk population during the whole process of the rainstorm flood disaster.