In order to enhance the encryption speed and secrecy of the current chaotic secure communication system,a scheme of digital secure communication based on the stream cipher produced by real-time synchronization is presented. A improved cellular automata is used for encryption in the secure communication scheme while the expanded key technology and confusion technology are adopted. Numerical results show that the secure communication scheme can produce good pseudo-randomness stream ciphers while it has very fast speeds of encryption and decryption and high anti-attack ability.