Abstract: Dual-hosts sytem providing high availability(HA), which is the one of most important method for data integrity and service continuous, is widely required for user.The principle, deployment and framework of a dual-hosts system is introduced, and a turing machine of the dual-hosts system is presented in this paper.The turing machine simulates the dual-hosts system and describes the states transition of the dual-hosts system and supplies a theory basis for designing architecture of the dual-hosts system.