Addressing the limitations of traditional bidirectional CLLC resonant converter design methods—which only perform qualitative analysis of full-load efficiency and overly rely on empirical resonance frequency selection—this paper proposes a parameter optimization design approach based on an efficiency model. First, the converter's operating characteristics are analyzed using fundamental frequency approximation, establishing analytical constraints for peak gain, gain linearity, and Zero Voltage Switching(ZVS ) with respect to parameters K and Q. Subsequently, through time-domain analysis, an accurate efficiency model is constructed using parameters K, Q, and R_o, comprehensively accounting for switching losses and magnetic component losses under various loads. Subsequently, within the feasible design constraints, the efficiency model enables optimal converter design across a specific frequency band. Simulation and experimental results demonstrate that the proposed method achieves high efficiency and power density while satisfying voltage conversion requirements and enabling soft switching of components.