On the basis of set theory and graph theory, this paper defines some conceptions of business flows, and proposes a requirements description model based on conditional directed graphs (CDGRD). The model focuses on business flows that are described by means of conditional directed graphs. A business flow can be described by some rectangles (activities) and some lines with arrowheads (conditions), the description approach distinctly illustrates the logical relationships among business elements, such as diversified static data, dynamic data and processing flows, which are used in the course of every business implementation. Finally, an application instance of the presented model is given in a large-sized enterprise information system project.