Abstract: The experimental system was constructed via an AC ASMT welding system, a high-speed imaging system with synchronized waveform information acquisition and an infrared thermography camera. During the welding process of 2A12 aluminum alloy under different EP/EN cycles, the droplet transition, temperature field and influence laws of weld formation were investigated. The results show that the welding processes under different EP/EN cycles remain relatively stable without spatter. During the AC ASMT welding process, the "bell-shaped" arc semi-encloses the droplet at the EP stage. At the EN stage, the arc spot climbs up along the welding wire, accelerating the melting of wire and generating large droplet transitions which can oscillate and stir the weld pool. Additionally, the weld temperature during the EN stage is approximately 50% lower than that during the EP stage, realizing the decoupling of welding current and temperature. At an appropriate EP/EN ratio of 3:3, the weld with good appearance can be obtained. In addition, the weld reinforcement of 3.15 mm and the weld width of 5.04 mm are between those of DC EP and DC EN methods.