A novel high voltage gain DC/DC converter with three phase cascaded structure is proposed in this paper. An integrated magnetic, which integrates six inductors and three transformers into one magnetic module, is employed for the three-phase circuit integration. The integration method realizes high voltage conversion and high-power density of the converter. CLLC topology is used for each phase. The working principle of three phase CLLC circuit is analyzed. The magnetic integration design method is introduced. The design method of the leakage inductance of the integrated magnetic is derived through magneto resistance analysis. A 10 kW circuit prototype has been developed. Experiments have been carried out to verify the above proposed methods. The power density of the prototype circuit is 5.34 W/cm³, and the maximum efficiency is measured as 98.31%.