Abstract: To meet the needs of the wireless transfer system for the high-efficiency high-power millimeter wave power source, the high-efficiency technology research of high-power continuous wave klystron was carried out, and the efficiency of the klystron was effectively improved by using the depressed-collector technology. This paper mainly introduces the design scheme of the depressed collector of a Ka-band high-power CW extended interaction klystron (EIK), including the investigation of electron energy distribution and behavior characteristics, the setting of the initial condition, the structure and the setting of the electrode voltage of the collector, and the design and calculation results of a single-stage depressed collector and a two-stage depressed collector for this high-power EIK. The results of PIC show that the recovery efficiency of the EIK with single-stage and two-stage depressed collector are 41% and 68.8% respectively, the net power conversion efficiency of this EIK is raised from a base value of 27.5% to 54.8% by using a two-stage depressed collector, which shows that it is feasible to improve the efficiency of high-power klystron by adopting the depressed collector technology.