Abstract: Plasma arc welding have a potential application in welding middle-thickness work-piece. The gas focusing plasma arc welding system is self-designed and constructed. And gas focusing plasma arc welding is initially studied by welding experiment in this paper. The weld is detected and analyzed by the welding monitoring system. The thermal scanning picture can reflect the real weld formation almost. With welding current as 150 A, the weld depth is 6.53 mm in conventional plasma arc welding SUS 304 of 8 mm thickness, while the work-piece is fully penetrated in gas focusing plasma arc welding. Both up weld and down weld are good in gas focusing plasma arc welding. The experimental data shows that arc voltage in gas focusing plasma arc welding is higher than that in conventional plasma arc welding with the same welding current. It is found that gas focusing plasma arc welding can improve penetrability.