In order to improve the competitiveness and economic benefits of enterprises, the use of network technology for business processing is the inevitable trend of social development. Network security issues attract more and more attention. This paper takes the small and medium-sized enterprises as example, analyzes the relevant theories and technologies of network security, discusses the network security requirements of small and medium-sized enterprises, and clarifies the whole network structure and network topology of small and medium-sized enterprises. And then a more complete enterprise network topology is designed. Through the reinforcement on the physical environment security, network border security, and the network security, the network security level of small and medium-sized enterprise is improved.