The complex network theory has been used to reveal some typical features of software networks. It provides a new way for us to understand the software structure from the system view. However, there exist some gaps between the theoretical results and the practical performance of software systems. This paper aims to reveal some essential causes for the above difference by analyzing the characteristics of software network nodes. This paper proposes a novel weighted network model to much more accurately describe the dependencies among the software network nodes. Based on this model, this paper analyzes the actual dependencies of the software network nodes and several statistical characteristics. Also, this paper further analyzes the relationships between these statistical characteristics and the nodes impact. Furthermore, this paper introduces the concept of the key node and four fundamental hypotheses. Finally, this paper verifies the effectiveness of the above four hypotheses by designing the experiments on two software systems. This study provides a guide to research in defects propagation, software reliability and software integration testing.