In software engineering, class diagrams are generally used to describe the relationship of classes. Software systems as networks are studied in this paper. By the demonstration and analysis of the large-scale software systems provided by Sun and IBM, it is found that the degree distribution of software systems written in Java is characterized by the scale-free distribution, and its scaling exponent γ is about 2.5. In the evolving process of software systems, in addition to addition of nodes, there are some other local events as follows: addition of edges, random removal of edges and rewiring edges. The evolving model of software systems is established consequently. As for the network generated by this model, its degree distribution follows the power-law distribution. The actual application and numerical simulations validate this model.