In this paper, the effects of atmospheric pressure argon (Ar) plasma jet treatments on the surface properties of polyethylene terephthalate films are studied. Different discharge power density, treatment distance, and treatment time are selected to study the effects of discharge operation parameters on the surface treatments, and the surface properties of the untreated and plasma-treated samples are characterized through water contact angle and surface energy measurement, Fourier transformed infrared spectroscopy and X-ray photoelectron spectroscopy, with the modification mechanism being discussed. Results show that after the atmospheric pressure Ar plasma jet treatment, the water contact angle decreases with the increase of treatment time and surface energy increases with the increase of treatment time, and both of them reach saturated value at certain treatment time. It is found that, the shorter the treatment distance and the higher discharge power density are, the greater are the water contact angle decreasing and surface energy increasing, and the introduction of oxygen-containing polar groups is the main reason for the improvement of surface hydrophilicity. Therefore by shorting treatment distance and increasing the discharge power density, more oxygen-containing polar groups are generated in the jet, thus the treatment time to obtain the efficient treatment effect is reduced.