Abstract: For existing status of larger power supply load, power supply distance and active power loss of power supply system in coal mine underground and problems of low voltage and flicker at load side, modular multilevel converter-high voltage direct current(MMC-HVDC) system was applied to underground power supply system. Topology, mathematical model and parameters and controller design schemes of modular multilevel converter (MMC) in the system were mainly introduced. Underground MMC-HVDC system consists of two MMCs namely rectifier converter and inverter converter. Rectifier converter adopts double closed-loop control structure of fast current inner-loop control and constant DC voltage and constant reactive power outer-loop control, and inverter converter adopts constant AC voltage control. Both MMCs adopt carrier phase-shift PWM strategy and voltage equalizing strategy based on energy equipartition control and voltage balance control. Simulation experiments of the underground MMC-HVDC system were carried out in PSCAD/EMTDC software. The results show that the underground MMC-HVDC system has good dynamic response in steady-state operation, realizes independent control of active and reactive power, and improves DC transmission efficiency. When the system is disturbed by underground load variation, it can regulate active and reactive power rapidly and stabilize DC transmission voltage, and stabilize amplitude and phase of inverter converter output voltage at rated values.