The behaviors of the keyhole and the weld pool are dynamically-coupled in controlled-pulse plasma arc welding and can be used to indicate weld quality. The vision system was improved to detect the geometries of both the keyhole and weld pool at the backside simultaneously during a whole controlled-pulse keyholing period by one single CCD camera without auxiliary illumination. With the assistance of an appropriate optical filter system, the unchanged aperture and exposure time of the camera was also adopted and can be used under a different current period. An image processing method was then proposed to extract the keyhole boundary in open keyhole status and weld pool boundary for the whole welding process. The influences of current waveform parameters on the welding process are studied and discussed. It was found that dimensions of the weld pool at the backside are determined by the heat input of the plasma arc and keyhole duration. The keyhole moves forward during the welding process while the weld pool maximum width at the backside is located behind the keyhole center.