Abstract: In the process of variable polarity plasma arc welding (VPPAW), the state of the keyhole, particularly the back-side keyhole, was a critical factor influencing the weld formation quality. However, in practical welding operations, monitoring the keyhole state from the backside with sensors presented significant challenges. Therefore, this paper proposed a method for detecting the back-side keyhole on the front view using plasma arc welding based on the reflection from the front wall of the molten pool. With appropriate shooting angles, this method could simultaneously capture both the front-side keyhole images and the reflected shape of the back-side keyhole on the front wall. The relationship between the welding quality and the reflective shape of the keyhole was established through experiments at different welding currents. With the increase of welding current, the position of the detected keyhole image on the front wall was gradually shifted upward. When the weld appeared to be cut, the reflective shape of the keyhole on the back disappeared; when the stable weld state was disturbed, the reflective shape of the keyhole on the back changed at the same time. The stability of the keyhole was directly related to the stability of the welding states. The reflective shape of the backside keyhole on the front wall was affected by the welding state and the molten pool state. This provided important guidance for establishing the correlation between the reflective shape of the keyhole on the back and the welding quality.