A model was put forward to simulate the electromagnetic phenomena and fluid field in plasma arc occurring during the low-current microplasma arc welding (low-current micro-PAW) process. The effects of the nozzle neck-in and welding current of micro-plasma arc on the arc electromagnetic field distribution were discussed. Finally, under the condition of different welding current, welding voltage, arc length, shield gas flow rate, and plasma flow rate, welding experiments of image sampling were carried out. Three types of microplasma arc, namely, needle plasma arc, columnar plasma arc, and opening model plasma arc, are founded by experiment. Based on the unified model, a thorough investigation of the low-current microplasma arc characteristics during the micro-PAW process was conducted. It was found that the process parameters have significant effects on the microplasma arc and the distributions of current density and electromagnet force distribution.