Abstract: When the flexible DC power grid contains multiple voltage levels, the presence of DC voltage converter will complicate the stability analysis of the grid.Therefore, an impedance-based stability assessment method is proposed. Firstly, a unified impedance model of flexible DC power grid with multi-voltage levels is established, which contains modular multilevel converters (MMC) with different control modes. Based on this model, the transfer functions between different variables in the system are analyzed. All transfer functions contain an equivalent loop gain, thus the stability of DC power grid can be judged by whether the loop gain satisfies the stability criterion or not on the premise that the converter operates stably alone. Finally, based on the MATLAB/Simulink, a time-domain simulation system of flexible DC power grid with multi-voltage levels is built. The simulation results verify the correctness of the proposed small signal modeling and stability analysis method, which provides a theoretical basis for the future deployment and analysis of DC power grid.