Efficiency is one of the important technical parameters of Traveling Wave Tube(TWT). In order to improve the efficiency of a 0.22 THz folded waveguide TWT, it is necessary to design a multistage depressed collector. Firstly, the spent-electron-beam characteristics in the collector entrance are analyzed, and the voltage settings at the highest collection efficiency are predicted. Then the structure and voltage settings of the three-stage depressed collector are optimized by electromagnetic simulation software. The axisymmetric three-stage depressed collector with collector efficiency greater than 87.5% and back streaming current less than 0.328 9 mA is obtained. Finally, an asymmetric three-stage depressed collector with a back streaming current less than 0.075 mA is obtained by using an oblique structure at the entrance of the second electrode. The results show that the back streaming current of 0.22 THz TWT multistage depressed collector can be effectively reduced by using the oblique structure.