This paper is involved in generation, explosion of cathode plasma of electron va cuum diode under four-pulse mode. Diode breakdown caused by cathode explosion is observed in experiment. On the assumption that the diode beams are space charge limited beams and according to the experimental data, the explosion velocity of cat hode plasma is 0.5—4cm/μs and is different from each other in differe nt pulse process. The effects of explosion of cathode plasma on diode voltage, dio de impendence and multi-pulse beams are analyzed too.