Abstract: By resorting to numerical simulation, an S-band split-cavity oscillator (SCO) is designed. The cathode model of explosive emission is built, and the SCO with a real diode is simulated. The typical simulation results are that when the electron beam voltage is 495 kV and the current is about 3.93 kA, an SCO can generate about 640 MW of HPM with a frequency of 2.85 GHz. The power efficiency is about 33.0%. At the same time, we also investigate the influence of the input voltage amplitude and its waveform, the anode-cathode gap and the cathode radius on the output high power microwave of the SCO. The preliminary results show that besides the rate of the grid, the diode impedance and the cathode radius have obvious influence on the output microwave of the SCO, the diode impedance is needed to match the voltage, and the triangle voltage will decrease the power efficiency of the SCO obviously.