Based on the dynamic behavior of the acousto optical bistable system driven chaotically, a secure communication system is proposed. It can be realized by adding message due to the phase property of the system and extracting message due to the chaotic signal produced by chaos driving at the receive end. It is found that message can be securely transmitted even the signal masked by large noise. The message recovery needs no chaotic signal perfectly synchronized with carrier but only depends on the correlation of transmitted signal to the output signal of driven system at the receive end. The scheme is praticable in application.