Abstract: Preliminary experimental research on a relativistic terahertz source with 0.14 THz based on the surface wave oscillator was carried out. The device adopts an overmoded rectangular slow wave structure for the sake of large power capacity and easy fabrication. The experiment was performed on the CKP1000 generator according to theoretical analysis and numerical simulation results. Power measurement uses the radiation power density integral method and frequency measurement employs the cut-off waveguide method. A terahertz wave with a frequency more than 0.136 THz, a pulse duration of 1.5 ns and output power more than 2 MW is obtained, under the condition of the electron beam voltage of 320 kV, the beam current of 2.1 kA, and the guiding magnetic field of 5 T.