Abstract: In this paper, a high barrier film of a certain thickness is deposited on the surface of the PET substrate to improve the barrier performance of PET films. The microwave PECVD technology was used to deposit SiO_x and DLC barrier films on PET substrates. The differences in structure, microstructure, and barrier properties of deposited films with different monomer ratios were studied, and the barrier properties of SiO_x/PET and DLC/PET composite films were compared. The results indicate that the different monomer ratios significantly affect the structure, microstructure, and barrier properties of SiO_x and DLC films. Under certain process conditions, both SiO_x and DLC films can fully exert their respective barrier effects, reducing the oxygen (O₂) transmittance of PET materials. DLC thin films were prepared using a mixture of acetylene (C₂H₂) and argon (Ar) gas. A small amount of Ar was beneficial for the dissociation of reactive monomers, while the etching effect on the surface of the film was weak. The film deposition speed was fast, and the surface particles were dense, which could effectively block gas permeation. The oxygen gas permeability of the DLC/PET composite film prepared can be as low as 0.58 mL·m⁻²·d, far lower than the 130 mL·m⁻²·d of unmodified PET film. SiO_x thin films were prepared using hexamethyldisiloxane (HMDSO) and oxygen (O₂) as reaction monomers. When the O₂ ratio was high, the bonding of silicon (Si) and oxygen (O) elements in the films tended towards a mesh-like and cage-like structure, increasing the difficulty of gas diffusion. The oxygen transmittance of the SiO_x/PET composite film prepared can be as low as 3.69 mL·m⁻²·d. This study proposes the application of microwave PECVD technology to deposit barrier SiO_x and DLC barrier films on PET substrates, providing useful references for the production process of high barrier PET composite films.