The quantum cryptography system has been proved to unconditionally secure based on Heisenberg uncertainty principle and quantum non-cloning law. Because single-photon are not easy to be generated and detected, and based on discrete variable it is difficult for the quantum key distribution to obtain high transmission rate of information. The paper puts forward that the two parts of communication get the same initial system parameters and state of chaotic systems from initial key; it can obtain the expanded key quantity. And the security of the scheme is discussed.