Abstract: The ratio of excitation inductance of the transformer to the primary resonance inductance - k has an important influence on CLLC resonant converter. Increasing k can reduce the resonance current and improve the efficiency, but the increase of k is limited by other factors. In order to increase k as much as possible and get the CLLC converter with the highest efficiency, the DC gain expression of CLLC converter was obtained by first harmonic analysis (FHA) in this paper. On this basis, two main factors that limit the increase of k were analyzed in detail. One was that too large k would make the gain curve non-monotonous in the working frequency range, which would lead to the converter unable to achieve stable control and lose zero voltage switch. The other was that the increase of k would reduce the maximum gain of the converter, which would make the converter unable to meet the voltage gain requirements. Through the mathematical derivation, the maximum ratio of the inductances under the restriction of the above factors was obtained, so as to improve the efficiency of the converter to the maximum extent. Finally, the effectiveness of the method was verified by simulation and experiment.