Multi-terminal hybrid UHVDC system can provide a larger capacity, more economical and more flexible transmission mode, taking into account the technical advantages of LCC and VSC. Compared with the two-terminal DC protection system, the DC line protection is the most different in multi-terminal UHVDC system. Hybrid modular multilevel converters (MMC) based VSC comprising of half-bridge (HBSM) and full-bridge sub-modules (FBSM) has the characteristics of fault self-clearance of HVDC lines. Therefore, the demands for DC line protection in this system are different from multi-terminal VSC-HVDC systems with HBSM topology. Based on the requirements for DC line fault identification and clearing of multi-terminal hybrid UHVDC transmission system, the key problems of DC line protection in the system are analyzed. DC line protection configuration and the coordination strategy of DC line protection are also proposed in this paper. Finally, based on PSCAD/EMTDC simulation, the feasibility and validity of the proposed scheme are verified.