Abstract: Metal particles, as the most detrimental contaminants to the liquid insulation of transformers, pose a significant threat when they accumulate within oil gaps. Due to their high conductivity, such accumulation drastically reduces the effective insulation distance of the oil gap, potentially leading to insulation breakdown under the sustained application of an external electric field. We established a test platform for the distribution and breakdown characteristics of metal particles in flowing transformer oil. Experiments were conducted under AC voltage to investigate the distribution patterns and breakdown behavior of metal particles in flowing transformer oil. Combining experimental data with simulations, the mechanisms underlying the influence of metal particle distribution characteristics on the breakdown voltage of insulating oil were discussed. The results indicate that, once the transformer oil begins to flow, particles become less likely to accumulate in high-field-strength regions. As the electric field strength increases, so does the concentration of particle accumulation. Moreover, the Weibull distribution curves of breakdown voltage shift leftward with the rise of the prestressing field intensity, with more pronounced shifts observed in the low flow velocity region. Simulations reveal the aggregation of metal particles between electrodes, the forces acting on these particles are further analyzed, and the mechanisms by which metal particle aggregation characteristics impact breakdown characteristics are clarified.