Atmospheric pressure plasma jet which can generate a plasma plume has good application prospects in wastewater purification, element detection, material treatment and so on, because the plasma plume is rich in abundant active species. In addition, the diameter of the plasma plume is usually small, which limits its work efficiency. In view of this, in this work, a large scale uniform plasma plume with a diameter of about 14 mm is produced in a plasma jet excited by an AC voltage in Ar at atmospheric pressure. The electron density and the concentration of oxygen atom as a function of different experimental parameters are studied by means of emission spectroscopy. Photoelectric measurement results show that the luminance of plasma plume increases when applied peak voltage or argon flow increases. There are two optical emission pulses per voltage cycle both in upstream and downstream regions of the plasma plume when the peak voltage is low, and the intensity of optical emission signal in the upstream is higher than that in the downstream. Both optical signal intensities of plasma plumes in the upstream and downstream increase with the increasing of peak voltage. There are three optical emission pulses at each voltage period in the upstream and downstream when the peak voltage is high. Regardless of the number of discharge pulses per voltage cycle, the optical emission signals for the upstream and downstream of the plasma plume are synchronous. OH, N₂, Ar, and O Ⅰ spectral lines can be observed from emission spectrum in the range of 300~800 nm both in the upstream and downstream discharge collected by a spectrometer. The emission intensity of Ar upstream is higher than that downstream, while the emission intensity of OH and N₂ is lower than that in the downstream. The electron densities for the upstream and downstream of the plasma plume are measured by spectral line intensity ratio. The results show that the electron density in the upstream discharge is on the order of 10¹⁴ cm^-3, which is higher than that in the downstream plume (10¹³~10¹⁴ cm^-3). In addition, the electron densities of the plasma plume increase with the increase of peak voltage and argon flow both upstream and downstream. In addition, The variations of concentration of oxygen atom with different experimental parameters are studied by using optical actinometry. It is shown that the concentration of oxygen atom decreases along the flow direction. For the plasma plume, the oxygen concentration increases with the peak voltage and the argon flow, averagely. The experimental phenomena mentioned above are explained qualitatively based on the theories of gas discharge.