Addressing the issue of significant phase errors in the secondary side current and reduced system transfer efficiency due to the neglect of higher harmonics when the operating frequency of the bidirectional half-bridge CLLC deviates from the resonant point and the parameters of the front and rear resonant cavities do not match, the traditional fundamental harmonic analysis method is used to calculate the phase of the secondary side current for synchronous rectification. This paper proposes to include the higher harmonics ignored by the traditional fundamental harmonic analysis in the calculation, using the extended harmonic analysis to equivalently represent each harmonic as an independent voltage source, and to correct the equivalent resistance of the load in conjunction with the law of conservation of power, further establishing an extended harmonic approximation model. According to the model, the effects of each harmonic acting individually are calculated, and the results of the actions of each harmonic are accumulated to derive the expression of the secondary side resonant current, from which the zero-crossing angle of the secondary side resonant current can be calculated, achieving accurate calculation of the phase where the secondary side current crosses zero. Finally, a 500 W rated power experimental prototype is built, and the experiment shows that compared with the traditional fundamental harmonic analysis method, the efficiency of the converter is improved across the full load range, with the highest efficiency improvement of 1.8% at the rated power, verifying the correctness and effectiveness of the proposed theory.