Windows 7 is a personal computer operating system developed by Microsoft Corporation, and it belongs to Windows NT family. This paper concentrates on the topic of analyzing the security level of Windows 7 Operating System, and security level of the information system highly depends on the operating system. To estimate the security grade of Windows 7 operating system, we use the graph model to describe the relationships between vulnerabilities. In the proposed graph model, AND Structure and OR Structure are exploited. To testify the performance of the proposed security evaluation method, we collect 1358 Windows vulnerabilities to construct a dataset. In our experiment, Windows 98, Windows 2000, Windows XP and Windows 7 are utilized, and three different types of user are applied, that is, distributed users, trusted users, and regular users. Experimental results demonstrate that Window 7 achieve higher security level than the former Windows version.