The medium-voltage (MV) power distribution networks have a complex topology, and this can easily cause air arc faults. However, the current of the air arc is low, and the arc temperature is only a few thousand Kelvin. In this case, the arc is in non-local thermodynamic equilibrium (non-LTE). The LTE state of arc is the basis for the establishment of arc model and the calculation of transport coefficient. In this paper, the non-LTE effect of the MV AC air arc is studied by the moiré deflection and the optical emission spectroscopy (OES) techniques.