Abstract: In the VPPA ( variable polarity plasma arc ) -MIG hybrid welding of aluminum alloy, when the welding current of VPPA is 130 A and the MIG welding current is 200 A, the MIG arc plasma is biased towards and connected with the VPPA plasma in the positive ( tungsten electrode is negative ) stage during the current base period, and does not connect with the VPPA plasma in the reverse polarity stage ( tungsten electrode is positive ), and the MIG arc voltage in the base stage is lower when the VPPA is positive. In order to clarify the above behavior mechanism, the hybrid arc was diagnosed by spectroscopy. Based on the Boltzmann diagram method and Stark broadening method, the average temperature and electron density of the VPPA-MIG hybrid arc at 2.5 mm above the test plate, the VPPA region, the coupling region, and the center of the MIG region are calculated. It is proved that the average state of the plasma at the above position is in a local thermodynamic equilibrium state. Based on the relationship between the emission coefficient and the plasma temperature, it is found that the high-temperature area of VPPA during the negative polarity period is larger than that during the positive polarity period by using Ar 794.8 nm narrow-band filtering combined with a high-speed camera. Moreover, the Al 396.1 nm spectral line in VPPA has higher radiation intensity and wider range during the reverse polarity period. It is proved that the decrease of MIG arc voltage in the base stage of hybrid welding mainly comes from cathode voltage drop rather than arc column voltage drop.