In order to improve the performance of traditional staggered double vane (SDV) traveling wave tube (TWT), a new kind of step-type staggered double vane (SSDV) slow wave structure (SWS) is proposed, and a new type of input/output coupling structure is designed based on this novel SWS. Moreover, a W band sheet electron beam SSDV SWS TWT is designed in this work. The calculation results show that the SSDV SWS TWT has a higher interaction impedance, so that it can achieve a higher saturated gain and higher interaction efficiency in a less interaction circuit length. In the frequency range of 90~100 GHz, the interaction impedance of SSDV SWS is greater than 4 Ω, which is higher than the traditional SDV SWS. The reflect ratio (S11) of high frequency structure of W band sheet electron beam TWT is less than -15 dB. Moreover, the saturated input power of the traveling wave tube is only about 0.7 W, and the maximum output power can be about 800 W, the corresponding efficiency is more than 7.8%, and the gain is more than 30.6 dB.