Abstract: A lot of GIS fault cases show that, after a period of live operation of the circuit breaker, the gap breakdown faults caused by particles often occur in the chamber. The impact vibration generated by circuit breaker operation can effectively stimulate the lift-off of latent metal particles in GIS, but the mechanism of gap breakdown induced by multiple particles during movement after lift-off is still unclear. In view of this, we carried out live operation experiments of withstand voltage and circuit breaker when multiple particles coexisted in the 252 kV GIS experimental platform to reproduce the normal operation and live operation process. The result shows that, in the long-term withstand voltage process, there is a greater probability that the moving particles in the circuit breaker will move to the end of the chamber and remain stationary, forming the aggregation of multiple particles. After the live operation of the circuit breaker, the multiple particles are excited by the impact vibration and jump, the series breakdown phenomenon is formed in the movement after the jump, and the breakdown voltage of the circuit breaker caused by the multiple particles during the live operation is always lower than the breakdown voltage during the withstand voltage. Based on steamer inception criterion, it is believed that, compared with the dispersive particles during the withstand voltage period, the aggregated particles formed before the live operation are closer to each other, and the distortion ability of the electric field is stronger during the movement process after the operation, so the series breakdown voltage of the particles formed is lower.