Abstract: The sheet beam klystron is a new kind of microwave vacuum device, which uses a high aspect ratio, thin rectangular beam to reduce space charge effects, the special rf structure to increase power capability and beam-wave interaction efficiency. By means of PIC simulation software, a 3D design platform of beam-wave interaction system was established, and applied to design and simulate the beam-wave interaction system of a W-band sheet beam klystron. Detailed parameters for the primary design were presented. The PIC simulation results indicate that output power of larger than 573 kW and gain of 27.8 dB can be obtained under the condition of 140 kV voltage, 15 A current. The efficiency exceeds 27.3%.