The polyethylene terephthalate (PET) was used for dielectric to produce the atmospheric pressure helium-argon mixture discharge plasma. The electrical and luminescence properties of PET dielectric barrier discharge were studied using a voltage probe, a current probe, a digital oscilloscope and a digital camera. It found that one or more current pulses appear in every half voltage cycle, and the discharge transits from uniform to pattern discharge with the increase of argon content. Argon atomic spectra intensities (696.54, 763.13, 772.09, 811.17 and 911.81 nm) were measured using a spectral system composed of the diffraction grating and a CCD detector. The influences of argon content and peak voltage (V_p) on the spectra intensity were researched. The results show that: at lower V_p, the above five argon spectra intensities enhance slowly, then weaken sharply, and enhance rapidly again with the increase of argon content; at higher V_p, the intensities of spectral line 696.54, 763.13 and 772.09 nm enhance and the intensities of argon spectral line 811.17 and 911.81 nm weaken. The discharge mode plays an important role in the spectra intensity variation at lower V_p, but the ionization mechanism makes a dominant contribution to the spectra intensity at higher V_p. At argon content ≤30% or ≥80%, the above argon spectra intensities almost remain unchanged, then increase to the stable value with the increasing V_p; at 30%≤argon content≤80%, the above five argon spectra intensities enhance slowly, then weaken sharply, and enhance rapidly again. The electron excitation temperature (T_exc) was calculated using the Boltzmann graph method, and the variation of T_exc with the ratio of helium to argon was obtained under different V_p. The results show that: the T_exc at high V_p is higher than that at low V_p, and the T_exc decreases with the increasing argon content. The reason is to maintain the balance between the ionization process and ion escape loss because the electron-helium collision section is much smaller than the electron-argon collision section, but helium has a larger diffusion coefficient than argon.