Compared to the traditional isolated bidirectional DC-DC converters (IBDC), the parallel-resonant isolated bidirectional DC-DC converter (PR-IBDC) has many benefits, such as high efficiency, low current ripple and high voltage gain. In this paper, a passive integrated unit with the flexible multi-layer foil (FMLF) integration technique is proposed to improve the power density of the PR-IBDC since many passive components are used in this converter for power conversion. Furthermore, the construction, flux analysis and parameter design of the proposed integrated unit are introduced in detail. Finally, a prototype and a simulation model based on Maxwell/Ansoft are built for a lkW-200kHz PR-IBDC to prove the validity of the proposed idea.