Abstract: The preliminary design of a Ka-band extended interaction klystron is presented with three-dimensional particle-in-cell(PIC) code. The extended interaction technology is widely adopted in resonant cavities used for high power, high gain, and bandwith applications. The electron gun and the focusing system are separately designed and optimized, where a permanent magnetic focusing scheme is introduced. The klystron is driven by a 0.15 A, 9 kV round-electron beam. An output average power of 355 W is achieved with an input power of 200 mW, corresponding to the gain of 32.5 dB , the efficiency of 26%, and the bandwith of 410 MHz at the centeral frequency.