The multi-chamber structure formed by connecting a plurality of semi-closed arc-extinguishing chambers in series divides the long arc flashed by lightning into several short arcs, and the high-speed airflow generated by the arc-extinguishing chambers has a strong arc blowing effect, which can effectively inhibit the burning of lightning flashover arc. In this paper, in order to improve insufficent performance of single arc-extinguishing chamber, the impact arc model in the arc-extinguishing chamber considering the external magnetic field is established, the movement characteristics of the impact arc in the chamber are studied and the effectiveness of the simulation modal is proved by quenching arc experiment. Based on the existing chamber structure, the movement characteristics of impact arc in semi-closed arc-extinguishing chamber and the influencing factors of external magnetic field acceleration effect are studied. The results indicate that the external magnetic field can accelerate the movement of impact arc in the chamber, and the stronger the magnetic field is, the more obvious the effect is. At the same time, optimizing the structural parameters of the chamber under the action of external magnetic field plays a positive role in improving the arc speed.