In this manuscript, weld pool dynamics in laser welding of various series of aluminum alloys were investigated by the in situ X-ray phase contrast imaging system. The experimental results showed that metal irradiated by laser was evaporated immediately, which generated the keyhole. Then metal surrounding the keyhole was melted gradually with the heat from keyhole. The growth rate of keyhole depth had a positive linear correlation with the total content of low boiling temperature elements (TCE), so did the keyhole depth and diameter at the stable stage. Longitudinal view area of the molten pool had a negative linear correlation with the thermal conductivity of aluminum alloy. The measured laser absorption rate had the same variation trend with the ratio of keyhole depth to diameter, and the highest absorption rate of 58% appeared in laser welding of aluminum alloy with TCE equal to 2.1%. Violent fluctuation in keyhole shape was avoided in aluminum alloy with TCE lower than 2.1%, where the surface tension and recoil pressure of metal vapor were balanced. To sum up, the effect of alloy element on weld pool dynamics in laser welding of aluminum alloys was firstly quantified in this manuscript.