With the implementation of energy strategy and the construction of a strong smart grid in our country, hybrid multi-terminal UHVDC transmission technology is rapidly developing and will be an important part of the China power grid in the future. Therefore, the accurate and quantitative evaluation of reliability of the hybrid multi-terminal UHVDC transmission system is of great significance to the safety and reliability of power grid. Based on this, the typical structure and operation modes of conventional HVDC systems and flexible HVDC systems are combed, and main wiring diagrams of hybrid multi-terminal UHVDC transmission system composed of aforementioned two systems are further studied. For each wiring diagram, according to the effect of main component failure and its position in primary connection, the HVDC system is divided into several subsystems such as AC filter subsystem and converter transformer subsystem, and then subsystems are modeled respectively. Finally, based on the state enumeration method, evaluation reliability model of hybrid multi-terminal UHVDC transmission system combined by the subsystem reliability evaluation models is established. Take Wudongde hybrid three-terminal HVDC transmission project as an example, the reliabilities under seven possible wiring diagrams in the project are evaluated and compared, the reliability sensitivity analysis of the failure rate of HVDC transmission line is also carried out.