Starch/polyvinyl alcohol (PVA)/clay nanocomposite films with high mechanical and barrier properties were prepared by extrusion blowing. Based on the starch/clay matrix, the effects of PVA content on the starch/PVA/clay nanocomposite films were investigated. Appropriate preparation conditions were determined by the rheological behavior of the blends. X-ray diffraction and transmission electron microscopy results demonstrated that the starch/PVA/clay nanocomposite films exhibited intercalated structures and the extent of intercalation increased with increasing PVA content. FTIR spectra showed that the molecular interactions were enhanced in the starch/PVA/clay films with higher PVA content. Dynamic mechanical analysis revealed partial miscibility between the starch and PVA in the films and the compatibility was improved when the PVA content was 50%. The maximum tensile strength and elongation at break for the starch/PVA/clay nanocomposite films were 30.18 MPa and 224.40%, respectively. The oxygen permeability of the starch/PVA/clay film with 50% PVA was dramatically decreased by about 210 times compared with that of a starch/clay film. A continuous phase inversion mechanism was proposed to explain the performance improvement of the starch/PVA/clay nanocomposite films. The starch/PVA/clay nanocomposite film is a promising material for high barrier food packaging. According to physico-chemical property changes and performance improvement of starch/PVA/clay nanocomposite films, a continuous phase inversion mechanism was proposed to explain the phenomena. Schematic representation of continuous phase inversion in starch/PVA/clay nanocomposite films. Display omitted •Starch/PVA/clay films with high performance were prepared by extrusion blowing.•The TS of 30.18 MPa and EB of 224.40% were obtained for the starch/PVA/clay film.•The OP of the starch/PVA/clay film is dramatically decreased by about 210 times.•The performance improvement was attributed to a continuous phase inversion mechanism.