Abstract: According to the strategy of controlled pulse key-holing, a new sensing and control system is developed for monitoring and controlling the keyhole condition during plasma arc welding(PAW).Through sensing and processing the efflux plasma voltage signals acquired during the welding processes with different levels of welding current, the quantitative relationship of the welding current, efflux plasma voltage and backside weld width of the weld is established.PAW experiments show that the efflux plasma voltage can reflect the state of keyhole and backside weld width accurately.Based on this, welding experiments under closed-loop control are conducted on ordinary plate and workpiece with varied thermal conditions, which have validated the stability and reliability of the developed keyhole PAW system.