A design method to enhance the efficiency of traveling wave tube is explored. The stair-step energy distribution of spent beam which is beneficial for the development of high collector efficiency depressed collector is obtained via the optimization of slow wave structures without degrading the basic electronic efficiency. A goal function expressed in the form of maximum collector efficiency of spent beam is designed to evaluate the collectability of spent beam. The optimization of slow wave structure based on collectability enhances the overall efficiency in the operation frequency band, and improves the small signal gain, but does not degrade nonlinear characteristic greatly compared with the simply optimization of slow wave structure based on electronic efficiency.