CLLLC resonant converters can realize bidirectional power flow and global soft switching, and are widely used in electric vehicle charging and DC microgrids. In this paper, static modeling method of CLLLC circuit are improved respectively. Since the state variable of the resonant converter contains high-frequency switching components and the average value of each switching period is 0, the condition of the small ripple assumption in the traditional state space averaging method cannot be satisfied. In this paper, the traditional Extended Describing Function(EDF) method is improved, and the corresponding state variables are represented by the superposition of sine and cosine components, and the weighted average of each mode is taken for each state variable. In the case that the switching frequency is less than the resonant frequency, in order to simplify the calculation process of the steady-state value of the state variable in the modeling and maintain high precision at the same time, this paper proposes a state variable optimization calculation method. Where, the equivalent output load introduces the information of switching frequency and resonant frequency, then the output voltage waveform is simplified and approximated to obtain more accurate output equivalent load. Thereafter, the optimized FHA model is obtained, which has the same simplified structure as the Fundamental Harmonic Approximation(FHA) method, and the problem of low accuracy solution of state quantities when it works under under-resonance is solved. Finally, the theoretical analysis and comparison are verified by simulation and experiments.