In the fields of using green energy such as EV power system and the PV system, the bi-directional DC-DC converter, which is used to control the charge and discharge of batteries, is now attracting more and more attention. In the bi-directional DC-DC converter for the battery charger, the response characteristics and stability of one are key points for the performance of the whole system. However, it is complex to analyze the dynamic characteristics and control design, because both of forward direction mode and reverse direction mode of the converter have to be considered. In this paper, the simplified dynamic model of the bi-directional DC-DC converter is presented. Transfer functions of the converter are derived by using the state-space averaging method. And, the frequency characteristics of the converter for both of buck mode and boost mode are discussed. Finally, experimental results are shown to confirm the validity of the proposed dynamic model of the converter.