Abstract: Impulse voltage conditioning is an effective method to improve the breakdown characteristics of the vacuum electrode gap. In this paper, spherical pure copper electrodes were subjected to impulse voltage conditioning using 64/700 μs impulse voltage. X-ray photoelectron spectroscopy was adopted to analyze the surface of the cathode, and the surface roughness and scanning electron microscope images of cathode were measured. Combined with the Fowler-Nordheim theory, this paper analyzed and diagnosed the breakdown mechanisms of vacuum breakdown waveforms in the process of impulse voltage conditioning, and revealed the evolution law of vacuum breakdown mechanisms. The results show that the comparison of Cu peak changes in the X-ray energy spectrum of the cathode surface, the surface roughness and its protrusion structure and the microscopic metal particles support and verify that the breakdown waveforms during the burn-in process include three breakdown mechanisms: pulse current-induced breakdown, field emission-induced breakdown and particle-induced breakdown, respectively. In addition, the evolution process of different vacuum breakdown mechanisms in the impulse voltage burn-in is as follows: pulsed current-induced breakdown and particle-induced breakdown dominate the beginning of the conditioning. Then, particle-induced breakdown dominates the burn-in, pulse current-induced breakdown disappears and the times of field-emission induced breakdown gradually increase. Finally, particle-induced breakdown and field emission-induced breakdown dominate the saturation stage of the burn-in, The experimental results provide a significant theoretical reference for the improvement of breakdown characteristics in vacuum electrode gap from the perspective of the evolution law of the breakdown mechanisms.