The devices in high v oltage direct current（HVDC） transmission system have complicated connection relations and their reliability levels are closely related to the reliability level of the system. In this study

the connection relations and functions of the devices are analyzed

and a fault logic diagram of HVDC transmission is established to determine whether the system operates with half capacity or bipolar capacity. Then the complex states transition processes of HVDC transmission system are analyzed and the state space transition diagram is established. The hybrid method is adopted to combine Monte-Carlo method with FD theory for the reliability evaluation. First

the probabilities of system states are obtained through Monte-Carlo method

then the frequency and duration are obtained through FD theory according to the transition diagram

so the curse of dimensionality caused by the complex states transition diagram is avoided. Example validates the effectiveness and correctness of the proposed method for reliability evaluation. Further

the sensitivity of the valve failure rate to the reliability of the example system

as well as the influences of different standby devices on the system reliability

are analyzed with the purpose of finding out the critical factor influencing the system reliability.