With the development of terahertz technology, high-frequency and high-power terahertz radiation sources have been the focus of researching all over the world. Based on the requirement of 0.8 THz terahertz wave imaging system, the regenerative feedback oscillator is designed and studied by using folded waveguide slow wave structure(SWS). In this paper, the SWS of 0.8 THz folded waveguide is designed and optimized by using the eigenmode solver in CST Microwave Studio. Then the oscillator is simulated and verified by PIC solver in CST Particle Studio. In the initial exploration stage of THz, regenerative feedback oscillator has the advantage of high feasibility and high output power. The simulation results show that a stable output signal of 60 mW can be generated.