Abstract: When the harmonic state space method is applied to a modular multilevel converter (MMC), the modeling of the control system and the electrical system is unified and integrated into the matrix A of the state space equation of the MMC system. This modeling method does not provide a convenient control system modeling interface, which is inconvenient to study the influence of the control link and is not conducive to the expansion of the model. This paper proposes a unified modeling of MMC DC side closed-loop impedance based on the harmonic state space. This method models the electrical loop and the control loop separately. The independent modeling of the control loop does not introduce new variables, and it can still easily analyze the influence of each control link and its parameters on the DC side impedance. The established impedance model has a clear structure, high accuracy, and few variables, which provides support for MMC DC side impedance analysis and parameter optimization. Simulation and experiment verify the accuracy of the established model.