Abstract: With the demand of satellite communication system for variable and efficient payloads, space traveling-wave tube (TWT) amplifier will be accelerated from single dedicated mode to power flexible multi-tasking mode. In order to solve the design difficulties of high efficiency power flexible space TWT under different output power conditions, a Ku-band helix radiation cooled space TWT has been studied. By improving the electron efficiency, collector efficiency and electron transmission rate under different anode voltages, in the 500 MHz bandwidth, the Ku-band TWT can generate continuous-wave output power over 150W and the overall efficiency greater than 68%. And the overall efficiency of the TWT is more than 60% when the output power back-off 3dB.