Abstract: A kind of W-band oscillator is designed. To reduce space charge effects, a thin rectangular sheet electron beam with a width-to-height ratio of 2 is adopted. In order to increase power capacity of the cavity, the periodic coupling cavity slow wave line is selected as the high frequency structure. And the ladder gradient rectangular waveguide is used as the output structure that can make the output structure and high frequency structure match well. The 3D-PIC simulation program is utilized to analyze the effect of the cycle number and length of the slow wave structure on conversion efficiency. The simulation results show that, when the cathode voltage is 13.0-16.1 kV, the high frequency structure can work normally, and when the cycle number is 9, the cathode voltage is 15.7 kV, the output power and conversion efficiency are highest,with the output peak power more than 1.9 kW, the average power of 980 W, the work frequency of 91.6 GHz, and the conversion efficiency of 25%. With the periodic coupling cavity length increasing, the work frequency decreases.