The DC/DC converter is an important component of the parallel-series plug-in hybrid electric vehicle (PHEV) and is also one of the most important electromagnetic interference (EMI) sources of the PHEV. This paper first analyzes the working principle of the step-down DC/DC full-bridge converter used in the PHEV and the main electromagnetic interference sources in the converter, Secondly, based on the radiated EMI model of Buck converter, the radiated EMI prediction model of DC/DC converter was established. The interference electromagnetic field strength was theoretically deduced, and the radiated EMI generated was simulated and analyzed on the HFSS simulation platform. On this basis, electromagnetic shielding and floating-point grounding measures were adopted to suppress the radiated electromagnetic interference of the DC/DC converter, and the electromagnetic fields before and after the suppression were compared and analyzed. The results show that the suppression measures proposed in this paper can effectively reduce the radiated electromagnetic interference in the DC/DC converter and thus meet the requirements of the vehicle.