Abstract: Metal droplet transfer mode which is influenced by the pulse parameters such as pulse current (I_p), pulse time (t_p), base current (I_b) and pulse frequency (f) has play an important role in the weld quality. An experimental system based on the Lab VIEW virtual instrument and high speed photography technology was developed to sample the transient electrical parameters and record the images of droplet transfer process simultaneously. A transition voltage U_dd in the pulse voltage drop edge of electrical waveform was discovered according to analysis of synchronous electric and image signals. The relationship between the metal transfer mode and the slope of the transition voltage was proposed.The result of experiments shows that the model can be used in closed-loop controlling of one droplet per pulse transfer mode and also can help for optimizing the welding parameters.