Following the growing research interests in complex networks, in recent years many researchers treated static structures of software as complex networks and revealed that most of these networks demonstrate small-world effect and follow scale-free degree distribution. Different from the perspectives adopted in these works, our previous work proposed software mirror graph to model the dynamic execution processes of software and revealed software mirror graph may also be small world and scale-free. To explain how the software mirror graph evolves into a small world and scale free structure, in this paper we further proposed a mathematical model based on the mechanisms of growth, preferential attachment, and walking. This model captures some of the features of the software mirror graph, and the simulation results show that it can generate a network having similar properties to the software mirror graph. The implications are also discussed in this paper.