Free metal particle defects are one of the frequent types of partial discharge defects in gas insulated switchgear (GIS) equipment, which have particle motion characteristics and cause more severe damage to the insulation medium. Accurate estimation of its size and severity of discharge is beneficial for understanding the insulation damage of equipment and providing targeted maintenance and repair guidance. This article proposes a method for estimating the internal metal particle size of GIS through digital virtual testing. Based on the particle coordinate sequence obtained from digital inversion, combined with the back-and-forth motion characteristics and simulation analysis of metal particles, the relationship between particle size and its motion behavior is established, thereby achieving online estimation of metal particle size and discharge capacity. The simulation study verified the feasibility and accuracy of the method proposed in this article, providing ideas for inferring the internal insulation status of equipment based on digital technology.