Abstract: The existence of DC component in the voltage on the side of converter valve will aggravate the accumulation of impurity particles, and the transformer oil flow will affect the accumulation characteristics. Therefore, using the built transformer oil circulating device, we carried out the accumulation experiments of free metal particles in static and flowing transformer oil at different voltages, particle concentrations and flow rates under DC field. The accumulation images of metal particles in the electrode gap was taken by a high-speed camera, and the change of leakage current during the accumulation process was recorded. The results show that the metal particles will accumulate in the center area of the gap in the both static and flowing oil flows, and the accumulated particles will constantly move back and forth between the electrodes. The number of metal particles accumulated in the center area of the gap and the value of leakage current through the gap both increase with the increase of voltage and particle concentration, but increase first and then decrease with the increase of flow velocity. Meanwhile, the low-speed oil flow (less than or equal to 0.02 m/s) will promote the accumulation of particles, while high-speed oil flow(higher than 0.02 m/s) will inhibit the accumulation. Combined with the force and motion characteristics of the metal particles in the oil, the trajectories of metal particles in static transformer oil were classified, the mechanism of the metal particles in the static oil to accumulate in the center area of the gap was clarified, and the influences of transformer oil flow on the accumulation characteristics of metal particles were analyzed.