Abstract: The influences of water vapor concentration on discharge mechanisms and discharge efficiency of atmosphere pressure plasma jet (APPJ) are studied to generate highly active cold plasma and optimize its efficiency. The discharge characteristics of APPJ with different water vapor content are studied by means of electrical measurements on voltage and current waveforms and Lissajous figures, and the optical properties are diagnosed using optical emission spectra and light-emission pictures. The main discharge parameters, such as discharge power, transported charges, electronic excitation temperature, molecular vibrational temperature and molecular rotational temperature are calculated, and the changing tendencies of them with applied voltage are studied, with the experimental results being explained by analyzing the discharge mechanism. The results show that the APPJ in Ar/H2O produces N2, Ar, O, and OH. Gas temperature changes between 525 K and 720 K, which shows that the APPJ in Ar/H2O plasma is a typical non-thermal one. With the increase of water vapor concentration, the plasma plume, light intensity and discharge power decrease, and the rotational temperature and vibrational temperature increase. Under certain discharge power, the content of OH produced reaches maximum when the water vapor content is 0.5%. Accordingly, better results can be achieved under this condition for application in surface modification of polymer materials.