A slotted rectangular single-grating with a cylindrical electron beam was proposed as the beam-wave interaction circuit of a W-band backward wave oscillator (BWO). Analysis on the slow-wave characteristics of the structure was done utilizing three-dimensional electromagnetic field simulation software CST-MWS. Results are as follows: The new structure can have a much larger coupling impedance than traditional one; the bandwidth of the fundamental mode can be broadened and the fundamental mode is unlikely to compete with the high-order mode. The loss caused by the skin effect is reduced. The structure was applied to design a W-band backward wave oscillator whose band center is 94 GHz. A simple slow-wave transition part, and the output coupler and terminal matching attenuator were designed, the parameters of which were optimized to obtain good signal transmission. Using CST-PS’s PIC solver, a three-dimensional large-signal particle simulation was done. After setting a suitable electron current and other parameters, watts scale peak output power was obtained within a wide frequency band by adjusting the working voltage, and the electronic efficiency in the band was greater than 1%.