Equivalent converter circuit/mathematical models are effective design guidelines and tools for controller. In this paper, a simplification small signal equivalent circuit mathematical model based on extended describing function method for LCLC resonant converter is proposed. According to the characteristic that the converter resonant capacitor has an equivalent resonant inductance about the modulation frequency, the original ninth-order circuit model is simplified and the small signal equivalent circuit model is derived by Thevenin–Norton branch equivalent transformation method. The simplified equivalent circuit mathematical model can predict the beat frequency characteristic and dynamic behavior very well. Furthermore, the transfer function analytical expressions for input-to-output voltage and input impedance are built, which is beneficial to the design of closed-loop feedback controller. Finally, the correctness of third-order simplification model is verified by Simulink simulation results.