Abstract: This paper discusses a kind of finite automaton cryptosystem.We introduce a software implementation of this cryptosystem on microcomputer.The speed of encryption and decryption is rather fast,for example 24000 bps in Z-80 microcomputer.And you can get more fast rate if you use fast machine such as IBM PC. This finite automaton cryptosystem has a strong security,and it can resist statistic attack and machine identification attack.