In order to have abetter understanding on the discharge mechanism of Ar/CH4 plasma jet and the state of its internal electrons under atmospheric pressure, a stable Ar/CH4 plasma jet was produced in the atmosphere through a self-designed needle-ring Dielectric Barrier Discharge structure with a discharge frequency of 10kHz, and the plasma jet was diagnosed by emission spectroscopy. The discharge phenomenon of Ar/CH4 plasma jet and the types of active particles inside were analyzed under atmospheric conditions. The effects of different argon-methane volume flow ratios and different peak voltages on the electron excitation temperature, electron density and concentration of CH group active particles in atmospheric pressure Ar/CH4 plasma jet were emphasis studied. The results show that the Ar/CH4 plasma jet is light blue under atmospheric pressure, and filamentous burrs can be observed on the edge of the jet. A harsh ionizing sound accompanies the discharge, and the shape of the jet tip fluctuates greatly; The main active particles in Ar/CH4 plasma jet at atmospheric pressure are CH group, C atom, CⅡ, CⅢ, CⅣ, ArⅠ and ArⅡ. Among them, the spectral lines of carbon-containing particles are mainly concentrated between 400 and 600 nm, and the spectral lines of ArⅠ and ArⅡ are distributed between 680 and 800 nm; It can be found that the concentration of the CH group increases with the increase of the peak voltage, however, the concentration of CH group decreases with the increase of the volume flow ratio of Ar/CH4. At the same time, the concentration of C atoms in the Ar/CH4 plasma jet increases. This means that the increase of the volume flow rate accelerates C—H fracture in the Ar/CH4 plasma jet. Therefore, it can be seen that increasing of the peak voltage and Ar/CH4 volume flow ratio can significantly accelerate the dehydrogenation efficiency of methane molecules, but increasing the Ar/CH4 volume flow ratio dehydrogenation effect is more obvious. Four ArⅠ lines was selected to calculate the electron excitation temperature under different operating conditions. The electron excitation temperature of Ar/CH4 plasma jet is between 6 000 and 12 000 K, and it shows an upward trend with the increase of the peak voltage and the volume flow rate of Ar/CH4; The electron density of the Ar/CH4 plasma jet was calculated based on the Stark broadening mechanism, the magnitude of electron density can reach 1017 cm-3, and plasma jet electron density can be significantly improved by increasing the volume flow ratio of Ar/CH4 and peak voltage. The exploration of these parameters is of great significance to the study of atmospheric pressure plasma jets.