The invention discloses a reverse working circuit of an LLC resonant converter, and a design method. The reverse working circuit comprises two full-bridge circuits, a resonant capacitor Cr, a resonantinductor Lr, a magnetizing inductor Lm and a transformer T. The full-bridge circuits are respectively connected to the primary end and the secondary end of the transformer T in parallel, and the full-bridge circuit at the primary end of the transformer T is connected with a power supply Vin; the resonant capacitor Cr, the resonant inductor Lr and the magnetizing inductor Lm are connected at the secondary end of the transformer T in parallel, and the full-bridge circuit at the secondary end of the transformer T is connected with a load Re. The design method includes the steps of determining the transformer ratio N and the circuit quality factor Q, determining the value of the inductance ratio Ln, and further determining a circuit frequency modulation range; acquiring maximum and minimum gains of the converter; judging whether the voltage gain meets the design requirements or not; finally determining the resonant capacitor Cr, the resonant inductor Lr and the magnetizing inductor Lm. The circuit has the advantages that soft switching can be realized, the power density of the converter can be effectively improved, and the application range is wide.