In the islanded DC microgrid, the voltage oscillations are prone to occur under the sudden change of the constant power load because the low inertia and weak damping characteristics of the power electronic and the stochastic fluctuation characteristic of renewable energy sources. In this paper, the small signal model of the islanded DC microgrid is established. And the impedance-based analysis method is used to reveal the destabilization mechanism that lead to voltage oscillation. First, the controller parameters of bidirectional DC/DC converter are designed. Furthermore, to suppress the voltage oscillation in the DC microgrid, a controller parameter optimization method is proposed from the perspective of the overall interconnected system. Finally, the simulation results demonstrate that the proposed controller parameter optimization design method is an effective way to suppress the voltage oscillation.