This paper proposes a multi-resonant DC-DC converter based on switched capacitor pre-buck. The converter can realize soft switching in the full load range and has good voltage gain characteristics. In order to effectively reduce the voltage stress on the device, the front stage of the converter adopts a stacked structure of switched capacitors. This structure realizes the function of high step-down ratio by pre-stepping the bus voltage. After reasonable parameter design and frequency matching, the third harmonic is superimposed on the high-order resonant link of the latter stage during the energy transmission process. This will significantly reduce the loss of the secondary side rectifier diode and increase the efficiency. This paper introduces the working principle of the converter, proposes a five-element parameter design scheme, and analyzes the parameter dispersion to assist the parameter design scheme to make it more reasonable, and finally conducts experimental verification. A 100 W prototype with 400 V input and 12 V output was built in the laboratory to verify the correctness of the theoretical analysis.