The modular multilevel converter (MMC) has become a very popular topology in the field of HVDCs. A decoupling control strategy is proposed due to the advantage of controlling the ac and dc loops independently. In addition, MMC impedance model under decoupling control is established to analyze the advantage of perturbation suppression in comparison with traditional control, which regulates dc voltage by adjusting ac current. The accuracy of the established impedance model and effectiveness of decoupling control strategy are verified by simulation and experimental results.