With the rapid development of quantum computing technology, encryption technology based on computational complexity is facing a huge threat, and single firewall technology cannot completely prevent hackers from intruding and attacks from the Intranet. The strategy which combines the firewall and quantum private communication technology can solve the above problems effectively. This paper designs a file security system in LAN based on quantum key, using quantum key to encrypt the internal privacy files of the LAN and one-time-pad encryption method can effectively prevent the loss caused by file stolen. The system is composed of two parts:the client software with the file processing unit as the core, and the server with the encryption/decryption information management module as the core component. In order to increase the complexity of ciphertext, the security of key store and the calculation feasibility, this paper proposes three key processing methods:mixed ciphertext splicing, misplaced mixing of weak password and intelligent matching of quantum key, which effectively improve overall security of the system. The effectiveness, security and feasibility of the system are verified by the system function test and network simulation analysis.