Abstract: The case study on hierarchical structure of large-scale open-source software shows that software systems are reflecting characteristic of flat hierarchical structure. On this basis, we took advantage of k-core to divide the software system structure into layers, abstract the core frame structure (CFS) of software system. The statistics on weighted connected degree between CFS and other layers show that CFS tightly communicates with other layers and its nodes have great influence on the nodes of other layers. Then, through the metrics on network parameters of CFS, It was found that CFS reflects characteristic of free-scale and small-world, and reflects higher degree of software reuse. In addition, CFS is in a decisive status in the software system integrated structure.