The LLC resonant converter (LLC)possesses the features of high efficiency, high power density and galvanic isolation, which makes it a promising candidate for the energy storage converter in DC microgrid applications. In DC microgrid, power electronic loads are widely used, such as motor drivers. The negative impedance characteristics of these constant power loads (CPL) will affect the stability of the source converter. In this paper, a control method is proposed for the LLC resonant converter in the energy storage system. By adopting the nonlinear disturbance observer (NDO), the negative impedances caused by the power electronics loads are compensated. In addition, a reduced-order model for the LLC resonant converter is used to simplify the controller design. A Lyapunov based stability analysis is carried out, and the simulation results of using Matlab/Simulink are given to show the advantages of the proposed control method.