A novel PSOI (partial silicon_on_insulator) high voltage device with double-faced step buried oxide is proposed, which is called DSB PSOI(PSOI with double_faced step buried_oxide layer). The surface electric field has ideally uniform distribution due to the additive electric field modulation by double step buried oxide. A silicon window underneath the source helps to reduce self-heating.The depletion region spreads into the substrate and the vertical electric field in the buried layer is enhanced, which results in a higher breakdown voltage than that of conventional SOI device. A 2-D quantified optimal relation between the structure parameters is also obtained. The results indicate that the breakdown voltage of DSB PSOI is increased by 58% in comparison with conventional SOI, while maintaining the low on-resistance of the DSB PSOI device.