Abstract: A three-dimensional mathematical model is proposed to simulate the transient keyhole instability and weld pool behaviors in parallel dual-beam laser welding. It is shown that one or two keyholes might be created and the evolution of keyholes is complicated during parallel dual-beam welding. The development of keyhole depth could be characterized as three phases and periodic. The frequency of keyhole depth oscillation in parallel dual-beam welding is several kHz,which is in the same order of magnitude in single laser welding. It is also found that the amplitude of keyhole depth oscillation decreases as the welding speed increases,and the keyhole could even be stable if the welding speed is large enough. Moreover,it is shown that the amplitude of keyhole depth oscillation increases if the interbeam distance is too large or too small,and penetration fluctuation is found when the interbeam distance is too large.