Abstract: Abstract: The Modular Multilevel Converter (MMC) is a core component of High Voltage Direct Current (HVDC) transmission, offering numerous advantages. This paper establishes a mathematical model for MMC and constructs a multi-level control path encompassing "bottom-middle-top" layers, covering Nonlinear Modulation (NLM), capacitor voltage balancing, double-frequency circulating current suppression, and dual closed-loop vector control. Through MATLAB/Simulink simulation verification, this strategy achieves an output voltage THD of ≤3.2%, capacitor voltage fluctuation of ≤±2.7%, circulating current effective value of ≤42A, and dynamic response of ≤15ms. It effectively meets the high-precision and high-stability operational requirements of flexible DC transmission systems, providing support for engineering applications.