Abstract: With the merits of simple control strategy and high conversion efficiency, the recently proposed CLLC-type bidirection DC-DC transformer（CLLC-BDCT） has become an option for high-frequency isolation between the low voltage dc nanogrid and grid. However, owing to the complexity of operation behavior, CLLC-BDCT is hard to design and apply. This paper first pointed out the inaccuracy of the fundamental harmonic approximation（FHA） in the study of CLLC-BDCT. Specific operation mode description and analysis were carried out. Compared to FHA, the description and analysis not only revealed the diversity of operation modes but also made the exact prediction for the operation behavior and characteristic of CLLC-BDCT. On this basis, the correlations between the passive parameters and the zero voltage switching（ZVS）, DC voltage gain and conduction loss were respectively analyzed. With these analyses, the influences of the resonant elements on control stability and operational efficiency were clarified separately. And then an optimum design methodology for the resonant parameters of CLLC-BDCT was presented. At last, a 1.6k W CLLC-BDCT prototype with 380 V output and input was built. The experiment results verify the proposed analysis procedures and the design methodology for CLLC-BDCT.