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.