This paper proposes a new soft-switching strategy for an isolated Three-Port bidirectional DC/DC Converter (TPC). Two inner phase shift angles will be introduced to two secondary inverters to make the modulation more flexible. Unlike published researches on TPC, this paper analyzes the converter in the frequency domain based on the fundamental harmonic approximation method. The simulation results confirm that, the maximum error between the calculated and the simulation powers is 31.2 W in the scale of 500 W (6.24%). After that, a new modulation strategy is proposed to help achieve zero-voltage switching (ZVS) characteristics. As confirmed by experiment, ZVS is achieved for all MOSFETs. The maximum efficiency of 95.58% is recorded, whereas, in the investigated power range, the average efficiency is 94.66%.