Based　on　surface-wave　oscillator　(SWO),　the　high　power　0.34　THz　source　using　over-moded　structure　is　studied.　The　attention　is　paid　to　the　influence　of　the　parameters　of　the　slow-wave　structure　(SWS)　on　the　dispersion　curve,　and　then　the　SWS　is　optimized　According　to　the　simulation　results,　the　size　of　SWS　and　the　requirements　for　the　accuracy　in　the　SWS　machining　are　confirmed.　Finally,　the　source　is　simulated　by　the　particle-in-cell　method.　Numerical　results　show　that　the　structure　is　capable　of　radiating　a　terahertz　signal　with　a　frequency　of　0.34　THz　and　a　maximum　output　power　of　about　7.8　MW.　Moreover,　the　structure　works　in　the　state　of　an　SWO　stably.　The　research　of　the　SWS　is　the　foundation　of　the　design　of　0.34　THz　source,　and　it　is　also　very　meaningful　for　the　realization　of　the　source　in　engineering.