High-speed GMAW tends to be accompanied by periodic humping defects, thereby reducing the weld bead quality. A new method was proposed to actively control the weld pool flow for eliminating humping defects. A high-melting point solid pin was designed and inserted into the weld pool to stir the liquid metal during the welding process. The characteristics of the backward molten metal flow were extracted and compared by a high-speed camera. Combined with particle tracing technology, the momentum of the backward metal flow was calculated and analyzed, and the mechanism of hump suppression in high speed GMAW was further revealed. The stirring pin interacted with the liquid molten pool, resulting in a vortex zone behind the stirring pin, which significantly reduced the momentum of the backward molten metal flow, and thus it inhibited the formation of humping beads.