Abstract: Narrow welding process window often impedes plasma arc welding of medium - thick plate. In keyhole plasma arc welding (K-PAW), it is found that if the keyhole is not opened due to the weak ionic gas flow rate and the blind hole is formed, an elongated cavity parallel to the welding direction will be generated in the weld. When the cavity is formed, the morphology of plasma arc changes obviously. The arc area increases and fluctuates more violently, causing backward reflection. The formation process of elongated cavities was studied by high-speed camera. Experimental results show that the formation processs of elongated cavities is as follows: at a certain welding speed and a low arc energy, the dip angle of the front wall of the molten pool is enlarged, and the plasma arc reflected by the front wall will impact the back side, which would be bended and retained under solidification, then an elongated cavity exists. The study of elongated cavities is helpful to provide a new understanding of keyhole formation conditions and mechanism in keyhole plasma arc welding.