Polyethylene terephthalate (PET) emerges as the predominant packaging material for food and electronics. To prevent water vapor from penetrating the polymer, an effective method is to deposit a coating on it that does not damage it. Diamond-like carbon (DLC), which can be deposited at low temperatures, offers a promising protective coating. In this research, DLC coatings were chemical vapor deposited (CVD) on PET substrates using an anode layer linear ion source method. The findings indicate that the water vapor transmission rate (WVTR) of PET substrates coated with DLC coating was significantly reduced to as low as 0.12 g/(m²·24 h), which represents a 30 times reduction compared to the uncoated PET substrates. To further understand the mechanism of water vapor barrier capabilities, molecular dynamics (MD) simulation and first-principles density functional theory (DFT) calculation were carried out to verify the capability and active site of the adsorption and dispersion of water molecules on carbon coating and PET. In addition, the DLC coated PET exhibited exceptional flexibility, and even after subjecting it to 2160 bending cycles, it showed no signs of cracking, indicating its mechanical robustness and durability. This research serves as a strong reference for utilizing DLC to address water vapor permeation challenges.