Gas-insulated switchgear (GIS) has been widely adopted in ultra-high voltage transmission and offshore wind power systems owing to its high capacity, reliability, and environmental adaptability. Nevertheless, metallic particle contamination poses a critical challenge to the insulation performance of GIS equipment. This paper investigates the influence of metallic particle geometry, dimensions, and positioning on electric field distortion across basin-type insulator under DC electric field through an AC GIS coaxial cylindrical test platform. Experimental results demonstrate three principal findings: (1) Linear particles induce the most severe electric field distortion, followed by flaky particles, with spherical particles exhibiting the least impact; (2) The distortion effect of spherical particles generally intensifies with increasing radius but shows reduced maximum field strength at 1.5 mm; (3) Field distortion initially escalates with particle length before stabilizing beyond 4 mm. Furthermore, particle orientation significantly affects field distortion, with maximum distortion occurring when the particle’s longitudinal axis aligns parallel to the electric field direction. These findings provide critical insights for optimizing GIS insulation design and developing particle suppression strategies, offering practical significance for enhancing GIS operational reliability.