Polylactic acid (PLA) packaging materials are useful, safe, and degradable in the natural environment, and hence, have attracted considerable interest. In particular, the barrier properties of PLA are the focus of current research. In this study, organic montmorillonite (OMMT) modified with an epoxy chain extender (CE) was prepared as a barrier functional additive for PLA and PLA alloy materials. The effects of the CE‐OMMT on the properties of PLA and PLA/PHA (polyhydroxyalkanoates) composites were investigated. The CE‐OMMT was characterized using Fourier transform infrared (FT‐IR) spectroscopy, X‐ray diffraction (XRD), and thermogravimetry (TG). The results showed that the CE‐OMMT was successfully synthesized, and that the addition of the CE did not change the crystal structure of OMMT. The morphology and microstructure of the PLA/CE‐OMMT and PLA/PHA/CE‐OMMT composites were characterized using XRD and transmission electron microscopy (TEM). The dispersion of the CE‐OMMT in the PLA and PLA/PHA composite films was significantly enhanced. XRD analysis showed that OMMT intercalated well into the matrix. The rheological, barrier, and mechanical properties of the PLA nanocomposite films were systematically studied. The results showed that the CE‐OMMT enhanced the mechanical and barrier properties of the composite films. Compared with neat PLA, the elongation at break of the PLA/CE‐OMMT and PLA/PHA/CE‐OMMT composite films with the addition of 3% CE‐OMMT increased by 449% and 788.4%, the oxygen permeability decreased by 57.74% and 59%, and the water vapor permeability decreased by 44.79% and 53.25%, respectively. Therefore, the modified PLA composite film is a promising substitute for packaging applications.