The passive start-up mode of a two-terminal MMC-HVDC with one terminal operating in islanded mode is introduced in this paper. The passive start-up mode includes the uncontrolled charging start-up stage and the controlled charging deblocking start-up stage. The equivalent circuit of the two-terminal MMC-HVDC in uncontrolled charging stage is established. The start-up process in uncontrolled charging stage is analyzed in detail, then the structure block diagram of MMC controller in controlled charging stage is given. Finally, based on RT-LAB real-time simulation technology, the mathematical simulation model of the two-terminal MMC-HVDC based on hardware in the loop is built, and appropriate MMC controller PI parameters are selected to simulate and analyze the passive start-up process of two-terminal MMC-HVDC. The effectiveness of the start-up mode is verified.