Abstract: Objective Variable polarity plasma arc (VPPA) welding is gradually receiving more and more attention as a high-quality method for efficient welding of aluminum alloys. The positive polarity (EP) stage arc characteristics are the key to determine the stable and defect-free welding of aluminum alloys, and the dynamic behavior of the cathode spot and its thermal effect are the most important features of the EP stage. This paper mainly focuses on the cathode spot motion law of VPPA and its influence mechanism. Methods The experimental platform is set up, the dynamic process of cathode spots is filmed by high-speed camera, and the image is processed by MATLAB to obtain a clear movement process of the spots, and its movement law is comprehensively analyzed with a single spot and spot clusters as the object of study respectively. Results The results show that the cathode spots are mainly located in the annular region 5～16 mm from the center of melt pool, and the new cathode spots are likely to appear in the vicinity of the previous cathode spots, which may be due to the plasma cloud formed by the metal vapors that prompts the emission of electrons. Taking an annular spot clusters with a width of 1 mm as an observation target, it can be approximated that the expansion rate from the center to the periphery is in the range of 1.75～2.25 m/s, and the velocity of a single spot is much larger than that of the outward expansion of this spot clusters. Conclusion The characteristics of spots with high current density and high heat have an important influence on VPPA welding formation of aluminum alloys.