It is an important content to evaluate the importance of network nodes in the research of complex network. A mathematical model for evaluating the key nodes of information system network is constructed in the light of the defects that node degree is unevenly emphasized and supporting function of edge to its correspondent nodes is ignored in the importance evaluation of complex network nodes. The concept of edge-betweenness is introduced in the person of the supporting function of edge to its correspondent nodes based on node degree, the mathematical model reflecting the joint function of node degree and edge-betweenness is formed. The simulation results by means of certain information system network demonstrate the effectiveness of the model for evaluating the importance of complex network nodes.