Under the action of controlled pulse current waveform, the welding current varies dynamically with the real–time key–holing situation, and there is a time lag between current signal and keyhole generation in plasma arc welding process, which is caused by the keyhole thermal lag effect. In this case, there are many pulse parameters like peak current level, duration and its dropping rate, which inevitably complicate the process. Determining the keyhole thermal lag time under various welding conditions is of great significance to ensure penetration depth and weld quality. In this paper, an experimental system for controlling pulse key–holing plasma arc welding has been developed, which measures both the welding current and efflux plasma voltage signals in real–time for characterizing the keyhole situation. The keyhole thermal lag time is defined as the difference between the instant at which the efflux plasma voltage is at 70% of its peak value and the intant at which the weldincurrent is at its peak level. The experiments are conducted to determine the valus of keyhole theral lag time by the in–rocess sampling during welding of constant–thickness and varied–thickness test plates of stainless steel. For constant–thickness ones, the average lag tme is 0.36 s, for varied–thickness ones, the welding current peak value is lowered with plate thicknss thinned gradually, but the keyhole thermal lag time is kept at an averaged value of 0.22 s with a lss deviation.