Thermal plasma of 12-phase AC arc was successfully developed and applied in the field of glass in-flight melting, and the arc discharge behavior was characterized by image analysis. The effects of sheath gas flow rate on arc discharge and melting behavior of granulated glass raw material were investigated. Results show that different sheath gas flow rates lead to various arc discharge and high-temperature region. The fluctuation of luminance area ratio and coefficient of variation reflects the change of arc discharge behavior. As the sheath gas flow rate increases, the ratio of luminance area decreases and the center temperature of arc increases. The vitrification degree of glass raw material is mostly dependent on the center temperature of arc, higher center temperature and more vitrification degree.