At present, stability research of modular multilevel converter (MMC) system mainly focuses on the traditional double closed-loop control, and extending this to other controls has become a challenge. To address this problem, firstly, an accurate impedance calculation method for MMC systems using embedded repetitive controller is proposed based on the multi-harmonic linearization method. The AC/DC side impedance model of the MMC is established, and accurate calculation of the AC/DC impedance of the embedded repetitive controller is achieved. Secondly, the influence of control parameters on the AC/DC side impedance characteristics of the MMC is analyzed. The effects of the proportional gain, internal mode filter, the gain of the embedded repetitive controller and other parameters on the stability of the system are obtained. Furthermore, the trends and extent of these parameters’ impact on system stability are analyzed, providing guidance for the parameter design of embedded repetitive control of grid-connected MMC systems. Finally, the stability of the system when connected to grids of different strengths is compared. Simulation results verify the feasibility of the embedded repetitive control system in weak grids, ensuring the stable operation of the MMC.