The non-isolated interleaving bi-directional Buck-Boost converter is faced with a widespread problem of low underloading efficiency. The Boost model of the multi-phase interleaving Buck/Boost converter is adopted as the research object in this paper and a comprehensive analysis of power loss is conducted under the DCM and CCM model. According to the principle that efficiency can be increased through reduced frequency during underloading operation of the transformer, and the principle of optimized frequency modulation is put forward to adjust the switching frequency under different loads so as to realize the goal of increasing efficiency. Concerning the defect of worsening ripples caused by reduced frequency, the magnetic integration technique is applied to the interleaving bidirectional Buck/Boost converter to efficiently improve the channel’s inductive current ripples and the output voltage ripples. Besides, the filter capacitor optimized design principle after magnetic integration is given an attempt to minimize loss and increase underloading efficiency. At last, the accuracy of the theoretical analysis is proved by simulation and experiments, thus providing new theoretical bases for underloading efficiency optimization of the interleaving bi-directional Buck/Boost converter and promoting realization of the full-load high-efficiency bi-directional Buck/Boost converter design.