The LLC half-bridge resonant converter has attracted extensive research attention due to its advantages of achieving high efficiency and high power density. However, the resonant process is more complicated, where the main difficulty is to precisely calculate resonant parameters. In this paper, the fundamental harmonic approximation (FHA) method is employed to analyze the resonant process of the converter. In particular, the working mode and the mathematical model of the LLC half-bridge resonant converter are established, where a new method is proposed to determine the operating frequency of LLC based on voltage gain and transformer loss. Based on the proposed method, the converter parameters can be optimized and the drawbacks of manually selecting the frequency can be avoided. Finally, a 200-W LLC converter laboratory prototype is established and the experimental results verify the effects of parameter optimization.