Abstract: Characteristic of laser beam-variable polarity plasma arc (LB-VPPA) hybrid heat source was studied for joining process of Al-Mn aluminum alloy. Arc shape was achieved by Y4-S2 high-speed camera, and welding current and arc voltage were obtained by Hannover welding quality analyzer. The results show that VPPA arc is attracted by laser and the mechanism which laser acts on arc is different for both positive polarity and negative polarity periods. Laser is concentrated on the roof of arc in positive polarity, while there is a protuberance-like plasma cylinder in negative polarity period. Roof plasma of VPPA arc absorbs laser by inverse bremsstroblung, and cross sectional area of arc becomes large as temperature increase. Meanwhile, laser provides plenty of free electrons to VPPA arc, thus current density and conductivity increase. Arc voltage decreases about 3-4 volts as resistance decreases. With the increasing of laser power, it obviously enhances the effects on VPPA arc.