During the transport., assembly., installation., and operation of a gas-insulated switchgear (GIS), the introduction of metal particles into the unit is inevitable. The presence of metal particles on the insulator surface is regarded as a critical factor in insulation failures. Thus, a thorough understanding of the characteristics of metal particles on the insulator surface is critical. In this study, a 126-kV GIS experiment platform with a highly sensitive partial discharge (PD) detection system is established. The movement and PD characteristics of micron-sized metal particles on the insulator surface in a 126-kV GIS under long-time alternating current (AC) stress are investigated. The results demonstrate that the PDs triggered by the micron-sized metal particles are weak and sporadic: the apparent charge of most PDs is lower than 1 pC, and the discharge repetition rate can be as low as once per minute. When two metal particles are sufficiently close to each other, they are charged by the PDs between them. This causes the metal particles to move away from each other and tend to be evenly distributed over the entire insulator surface. Some metal particles can move to stay at the triple junction near the grounded enclosure and center conductor or jump to adhere to the surface of center conductor. This study is anticipated to be a beneficial reference for further understanding the movement and PD characteristics of micron-sized metal particles on the GIS insulator surface.