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Molten pool oscillation behaviors during pulsed microplasma arc welding ultrathin stainless steel sheets were studied based on the numerical simulation, experimentally visual detection, and theoretical calculation. Without altering the average heat input, the effects of different pulse parameters of welding current on weld pool oscillation frequency and oscillation amplitude were discussed. It showed that the changes either in the weld pool temperature and liquid metal flow rate or in weld pool size cannot keep up with the rapid variations of welding current from base current to peak current or from peak current to base current. The mainly periodic changes of molten pool size and liquid metal flow rate are caused by pulsed current. However, on these main periodic changes, small amplitude fluctuations are superimposed due to weld pool oscillations. The oscillation frequency decreases and the oscillation amplitude increases slightly with the growth of molten pool. Oscillation frequency increased with an elevating in pulsed current frequency, and it rose firstly and then fell when duty cycle of pulsed current increased. Moreover, concerning the fluctuations in weld pool size, the oscillation amplitude reduced with the enhancement in pulsed current frequency though there were no obvious variations in it with the variations in duty cycle of welding current.