With China's steady progress towards the dual-carbon goal, photovoltaic, hydropower and other large-scale new energy power generation equipment will be integrated into the power system. However, due to China’s vast territory and the geographical distance between the power generation center and the load center, GIL have the advantages of large transportation energy and long transmission distance, and have become the first choice for transmitting electric energy. In recent years, GIL internal insulation faults have occurred frequently, among which are not a few caused by metal dust, and the size of metal dust is small, and the motion law is difficult to observe. In this paper, the diffusion and adsorption movement of metal dust in the air gap are studied. It is found that the unique diffusion phenomenon of metal dust in the air gap is accompanied by a large number of partial discharge phenomena. It is found that the discharge intensity is greatly related to the particle size of metal dust. The larger the particle size of metal dust, the more intense the discharge and the stronger the phenomenon. As the voltage increases, the diffusion and adsorption process of metal dust gradually ends, and the amount of metal dust adsorbed by the upper plate no longer increases, and the air gap breaks down. However, the breakdown voltage decreases by less than 10% compared with the case without metal dust, but there is a big difference in the amplitude of the discharge signal. The principle of affecting the amplitude of the discharge signal is that the lifting of the agglomerated metal dust will cause a large air gap electric field distortion in the air gap, and the size of the agglomerated metal dust affects the amplitude of the discharge signal of the metal dust.