With the growing of network information, the issue of information security has become the bot- tleneck of the development of network source application. This paper aimed at file's security in the computer and Internet, and researched on DES encryption algorithm and the algorithm of LFSR. Based on above algorithms, we designed a mixed encrypted method realized by FPGA; ac- cording to the password combination that is inputted by the user, a bunch of keys will be produced by LFSR and sent to computer by USB. The function of software on the computer is communicating with USB chip and encrypting files with DES algorithm. The results showed that this scheme has good effect and high processing speed on encrypting any certain files, and has a good effect for protecting files on network space.