To broaden the working range of on-board charger (OBC) for bidirectional charging and discharging and improve the charging efficiency, the power factor correction (PFC) cascaded high-efficiency CLLC resonant topology was used to realize the bidirectional conversion of energy between grid voltage and battery voltage. Meanwhile, in order to solve the problem of difficult to design parameters when CLLC resonant topology works in a wide range, a parameter design and optimization method was proposed. Fundamental harmonic analysis (FHA) was used for the modeling of resonant network. The normalized equations of charging and discharging gains were derived, and the key parameters affecting the gain and efficiency of the resonant converter were extracted. The influences of the changes in key parameters on the topology performance were compared and analyzed, and the parameter design and optimization principles were summarized. Experimental results show that the most important parameters that affected the bidirectional operation of the resonant converter were transformer turns ratio and resonant capacitance. The transformer turns ratio mainly affected the charging and discharging efficiency of the CLLC converter, and the value of the resonant capacitor affected the best working mode of the wide range output between the charging and discharging mode. The best value range of transformer turns ratio and resonant capacitor was given when the bidirectional transmission power was at the same level. By applying CLLC topology, the bidirectional on-board charger could achieve ultra-wide range operation. The charging efficiency of the whole machine reached 94.5%, and the efficiency of CLLC reached 97.5%.