Abstract: The profile change of molten pool during pulsed micro-plasma arc welding of ultra-thin sheets were collected by a backlight-based molten pool visual sensing inspection system. The influence of ratio of base current to peak current(0.29-0.71), duty cycle(30%-70%) and pulse frequency(25-100 Hz) on the weld molten pool size and weld quality of 304 stainless steel ultra-thin sheets under average current of 1.8 A was studied. The results show that under the experimental conditions, the weld pool width and area were less than 2.5 mm and 3.5 mm², respectively. The largest width and area of molten pool gradually decreased with increasing ratio of base current to peak current, duty cycle and pulse frequency. Three different types of welds, including discontinuous welds, continuously non-uniform width welds, and continuously uniform width welds, were obtained under different process parameters. The weld formation quality was the best and the weld was continuously uniform width weld under the duty cycle of 50%, the ratio of base current to peak current of 0.50, and the pulse frequency of 25, 100, 500 Hz.