A stable argon plasma plume is generated at atmospheric pressure by a plasma jet in dielectric barrier discharge configurations. The applied voltage, the current and the light emission signal are simultaneously recorded by an oscilloscope. Results indicate that the plasma plume is composed of successive fast-moving plasma bullets. Based on collisional-radiative model, the electron density of the plasma plume is investigated by optical emission spectrum in the range from 300 nm to 800 nm. Optical diagnosis shows that the electron density increases with the increasing applied voltage or gas flow rate. However, it decreases with the increasing driving frequency. The vibrational temperature and the rotational temperature of the plasma plume are also studied by optical emission spectroscopy. It is found that both of them increase with the increasing applied voltage or gas flow rate, and decrease with the increasing driving frequency. The aforementioned phenomena are qualitatively explained by analyzing the electric field of discharge.