The granulating is the segmentation from the whole into the parts. The elementary granulation and the granulation are defined in this paper, and with construction of the granular network, the relevant knowledge of graph theory is applied to granular network and its computing. In the paper it is shown using the example that it is feasible and effective that granular network is applied in describing data reduction in information system. The method has the characteristic of simple and visual form and so on. Compared with other analysis methods, its time complexity is decreased to O(n).