Owing to characteristics of open medium, dynamic topology and distribution, the security of Mobile Ad hoc Network (MANET) is more rigorous than that of traditional network. Referring to the I-ADD process of security analysis, the paper analyzed features, insecurity factors and security threats of MANET. Aiming at the security requirements, the design requirements for network security defense of MANET was brought out. Based on this and referring to OSI hierarchy model, the security architecture was designed. The analysis on relationship between each layer of the architecture and that of OSI was also provided, which offers framework for planning and designing secure and reliable MANET.