The work aims to obtain the SiOx/PET composite film with better performance by studying the effects of SiOx layer of different thicknesses on the mechanical properties and the barrier properties of SiOx/PET composite films, based on the excellent properties of silicon oxide (SiOx) layer. The SiOx layers with thickness of 40, 150, 230 and 320 nm were obtained by plasma enhanced chemical vapor deposition, with the self-made polythylene terephthalate (PET) film as the base material. Then, the Fourier transform infrared spectroscopy, and tests of mechanical properties and barrier properties were carried out, and the apparent morphology of the film was analyzed. After the SiOx layer was deposited, the tensile strength and elongation at break of the SiOx/PET composite films were increased first and then decreased with the increase in the thickness of SiOx layer, and the oxygen permeability and water vapor transmission rate were significantly attenuated and then gradually became gentle. The mechanical properties and barrier properties of the composite film were better when the thickness of SiOx layer was in the range of 150~230 nm. The tensile strength, elongation at break, oxygen permeability and water vapor transmission rate were improved by about 25.0%, 20.9%, 79.3% and 77.3%, respectively. The SiOx layer with suitable thickness can provide the SiOx/PET composite film with better mechanical properties and the barrier properties.