Aiming at the safety threat posed by metal particle contamination inside DC gas-insulated transmission line (GIL), where metal particles tend to adhere to the surface of basin insulators, a simulation model reflecting the actual operating conditions in DC GIL is established, and the impact mechanism of metal particles on the insulator's surface electric field is analyzed. In this paper, the millimeter-sized particle is selected, and the effect of various factors, such as particle size, shape, attachment location, and the aggregation of multiple particles, is researched on the electric field distribution of basin insulators. The findings indicate that the safety threshold of the insulator surface electric field strength is exceeded with a single metal particle adhesion defect. There is a boundary-diminishing effect of particle size on electric field distortion. The potential and curvature of particle attachment location are positively correlated with the degree of electric field distortion on the insulator surface. Furthermore, when particles are irregularly shaped, such as conical or linear, or when multiple particles aggregate longitudinally, there is a significant increase in the degree of electric field distortion on the insulator surface. The electric field distortion near the particles becomes more severe under the condition of voltage polarity reversal.