Plasma arc welding is widely used to weld stainless steel, aluminum alloy, titanium and other materials due to its excellent welding adaptability. In this study, infrared thermography and particle tracing were used to investigate the molten pool dynamic behaviors and the weld appearance defects in lap plasma arc welding. The particle tracing results show that when the gap is 0.3 mm or 0.6 mm, the liquid metal layer on the top plate is very thin, which solidifies prematurely, and a void defect is formed at periphery. When the gap is 0.9 mm, a larger solidification zone is formed on the top plate. The liquid metal at the front part of molten pool flows backward. And then the liquid metal is blocked by the solidification zone and gathered in front of the solidification zone. However, due to the wider gap, it is difficult to form a liquid bridge and a droplet is formed. After the droplet solidifies, humping is formed. The infrared thermography results show that the increase of the gap will form a low temperature zone in the middle of the molten pool, and the area of the high temperature will also increase, which are consistent with the results obtained by particle tracing.