In this paper, a composite passivity-based control method based on a finite-time disturbance observer (FTDO) and a passivity-based control (PBC) is proposed to improve the stability of the Boost converters with constant power loads in DC Microgrids. The FTDO improves the robustness and rapidity of the system by accurately estimating system disturbances. The PBC ensures the stability of the system via its transient energy dissipation. The FTDO operates parallel to the PBC, and compensates the observed value to the PBC through a feedforward channel. When compared with other controls, the proposed composite passivity-based control has the advantages of a fast dynamic response and accurate tracking of system disturbances in a wide working range. Finally, the control method proposed in this paper is verified by MATLAB/Simulink simulations and hardware in the hard-ware-in-loop experiments. [ABSTRACT FROM AUTHOR]