The network security is extremely important in the Internet environment. As cloud computing is more and more popular, and nowadays, it is ubiquitous in all kinds of Internet applications. At the same time, this new computing model has brought many new network security issues. In this paper, the authors study the network security model in the cloud computing environment. They classify cloud computing into three layers, and analyze their corresponding network security models. At last, they predict some challenges in the design and implementation of network security policies in cloud computing.