Abstract: Due to asymmetric structure on both sides of single side V-groove fillet weld of medium and thick plate, there is a great difference in heat dissipation during root welding, which is easy to produce incomplete fusion. In order to ensure a good penetration shape of root welding, the strategy of adjusting the welding torch angle in real time according to the changes of the front weld pool was proposed. The experimental results showed that the position of the torch shifted regularly relative to the welding pool with the change of welding angle. After median filtering and other processing on the acquired images of the front weld pool, the position coordinates of the welding torch center were obtained, and the internal penetration shape with different angles were studied. The quantitative relationship between the position offsets of the welding torch relative to the weld pool and the internal penetration shape were established. This provided a new method for predicting and controlling the internal penetration shape by using the feature information of the front weld pool.