Abstract: A optimal design of multi-stage depressed collector for K-band traveling-wave tube is carried out to achieve high efficiency and lower backward current. By utilizing the software MTSS, simulation result shows that: when the after-interaction beam enter into the collector, the efficiency of collector are 82% and 79.15%, respectively, for with and without considering the second electron. This results well fit the require of mm-TWT design, and the experiment test will be carried out.