Abstract: The dynamic of DC bus has large impacts on the impedance characteristics of MMC and the system stability in a high voltage direct current system. However, most of the current studies on MMC-HVDC system stability simplify the DC side as a DC voltage source, which will lead to the lack of practicality of the research results under certain operating conditions. First, a sequence impedance model considering DC voltage control is developed which can be used to illustrate the necessity of DC voltage control in the MMC-HVDC system stability analysis. Then, the influencing factors of DC voltage control parameters are discussed. The boundary conditions of DC voltage control parameters are given as well. Finally, two methods of MMC impedance optimization based on DC voltage control are proposed to improve the MMC-HVDC system stability effectively. The accuracy of the sequence impedance model and the effectiveness of impedance optimization methods proposed in this paper are verified in Matlab/Simulink.