A tunable feedback oscillator for 850 GHz terahertz wave imaging system is proposed. The use of a folded waveguide as the slow-wave structure permits the superior performance together with the capability of UV-LIGA process. The comparison of dispersion character of FWG designed and optimized using eigenmode solver in CST Microwave Studio, respectively applied in regenerative feedback oscillator and traveling wave tube shows the key factor of tunable frequency centered at 850 GHz. Additionally, a feedback circuit including T-joint structure with lossy metal is simulated and the design, including SWS and feedback circuit, is verified by 3-D particle-in-cell simulations. On varying the beam voltage, the frequency-adjustable oscillation changes from a stale single-frequency state at the beginning to multi-frequency spectra, demonstrating more than 200 mW output power.