Extended interaction klystron amplifier is a very important high power millimeter wave source with many actual and potential applications. Based on the electromagnetic simulation software and 3D PIC code, a W-band extended interaction klystron amplifier is designed. In the PIC simulation, when the beam voltage is 30 kV and current is 8 A, the device can generate a 43 kW output power at 96.8 GHz with an efficiency of 18% and a gain of 49.3 dB.