To address the limitation of insufficient elongation at break in cassava starch-based films, calcium ion-assisted pullulanase was used to enzymatically debranch cassava starch. Debranched starch (DBS) was blended with native starch to prepare composite films, and the effects of DBS on the mechanical and barrier properties of the films were investigated, along with the structural changes induced by calcium ion-assisted enzymatic debranching with pullulanase. Calcium ions enhanced the pullulanase activity, thereby improving the efficiency of starch hydrolysis and yielding DBS with an apparent amylose content of 73.21%, which was 30.71% higher than that of the calcium-free control. Structural characterization revealed that DBS granules transformed from a spherical to a flaky morphology. X-ray diffraction (XRD) showed decreased crystallinity and the emergence of a V-type peak. Fourier transform infrared spectroscopy (FT-IR) and proton nuclear magnetic resonance (¹H NMR) confirmed an increased amylose fraction and reduced branching degree. Incorporation of DBS into the films led to attenuation of the broad peak at 12~15° and enhancement of the new peak at 28~32° in XRD, while FT-IR exhibited weakened hydroxyl-stretching and water-band absorptions. Performance testing demonstrated that the composite films containing DBS maintained a tensile strength comparable to that of native starch films (33.16 MPa), while the elongation at break was significantly improved to 3.79%, representing a 28.23% increase. The water vapor permeability (2.76×10-12 g•m-1•s-1•Pa-1) remained unchanged, whereas the water contact angle increased from 63.1° to 74.2°. In conclusion, calcium ion-assisted pullulanase debranching effectively modulated starch structure, markedly enhanced film flexibility without the need for plasticizers, while maintaining film strength and barrier properties, providing a green and efficient strategy for reducing the brittleness of cassava starch-based packaging films.