For the first harmonic approximation (FHA), it is difficult to obtain accurate gain results when it deviates from the resonant frequency point, and it is not suitable for the application of wide gain range. Based on this issue, a new time domain analysis (TDA) method is proposed to derive more accurate gain expressions of CLLC resonant converters, so as to guide parameter optimization design. First, the deficiency of the FHA is analyzed. Then, the CLLC converters in different operating modes are modeled by TDA, and a more accurate gain expression is derived. Under the premise of satisfying soft switching, the inductance ratio k and the quality factor Q are optimized by the derived precise gain formula. The gain formula obtained by this method has high precision and a simple parameter design process. Finally, based on the parameters of the optimized design, a 1 kW experimental prototype is built that satisfies the wide gain range output. The simulation and experimental results both verify the accuracy of TDA and the feasibility of the parameter design method.