To　increase　the　generation　efficiency　of　the　terahertz　wave　in　the　Y　band,　the　idea　of　dual-reflector　is　introduced　in　the　relativistic　surface　wave　oscillator　(SWO)　with　large　oversized　structures.　The　dual-reflector　and　the　slow-wave　structure　(SWS)　construct　a　resonator　where　the　field　strength　of　TM＜sub＞01＜/sub＞　mode　inside　is　intensively　enhanced　and　then　the　efficiency　is　increased.　The　pre-modulation　on　electron　beam　caused　by　the　reflector　is　also　helpful　in　improving　the　output　power.　Meanwhile,　the　reflector　can　reduce　the　loss　of　negatively　going　electrons.　Through　the　particle-in-cell　(PIC)　simulations,　the　optimized　structure　is　tested　to　be　stable　and　little　power　is　transmitting　back　to　the　diode　area.　The　output　power　reaches　138　MW　in　the　perfectly　electrical　conductivity　condition　and　the　frequency　is　337.7　GHz　with　a　pure　spectrum.　The　device＇s　efficiency　is　increased　from　10.7%　to　16.2%,　compared　with　the　device　without　any　reflectors.　The　performance　of　device　with　lossy　material　is　also　focused　on.　In　the　situation　of　copper　device,　the　output　power　is　about　41　MW　under　the　same　input　conditions　and　the　corresponding　efficiency　is　about　4.8%.