Survivability helps ensure that underwater monitoring systems provide essential services under conditions of attacks, failures, and accidents. To evaluate the survivability of such systems accurately, definition of survivability and its model is proposed in this paper. Survivability, the ability of a system to meet user's requirement of service when the system is affected by some events in a specified environment, and is computed by mathematical expectation of services performance. This method will have more precise result than others in judging whether system design achieves the criteria of survivability. To the survivability model based on multi-state system, service performance is the function of state of all subsystems, which is related to structure function of system. Probability distribution of subsystem's states is decided by reliability and security. In the end, an illustrative example is presented to prove the effective of the survivability model is presented in the end.