Terahertz pulse radiation with nanosecond width and kilowatt level power generated by Relativistic Backward-Wave Oscillator(RBWO) has positive significance in terahertz technology applications. The particle simulation and design of a 0.34 THz-RBWO is presented. The sinusoidal period slow wave structure with an average diameter of 6.8 mm, a corrugation depth of 0.1 mm, a period of 0.26 mm is manufactured by fine fabrication processes, and the preliminary experiment of the 0.34 THz-RBWO is performed. The principles of measurement on signal width, frequency, power of 0.34 THz pulse are studied. The 0.34 THz-RBWO pulse radiation with a pulse width of 2.1 ns, a frequency range of 0.32 THz- 0.36 THz, and a power of 375 kW is measured through the preliminary experiment.