Amorphous calcium carbonate/chitosan (ACC/CS) nanocomposite films containing Mg-ACC (0-100 wt%) are prepared by simple natural evaporation-induced assembly. The influence of Mg-ACC content on the mechanical properties as well as cross-section of ACC-CS nanocomposite films were systematically investigated. The nanocomposite films exhibit an excellent mechanical property at Mg-ACC contents of 80 wt% compared to ordinary calcium carbonate/chitosan composites. With content of Mg-ACC increasing, the mechanical properties of ACCCS nanocomposite films were further increased, reaching a maximum when the content of Mg-ACC to 80 wt%. Strength and Young's modulus of the 80 wt% ACC-CS nanocomposite film reach 121.67 MPa and 31.96 GPa, respectively, which is 2.3 and 5.1 times higher than that of the pure CS film. After undergoing Mg-ACC crystal transformation when the Mg-ACC content is greater than 80 wt%, the smooth structure was transformed into crystal-like agglomerates, which deteriorates mechanical properties. Furthermore, the as-prepared nanocomposite films show excellent hydrophobicity and high biocompatibilities for boosting the growth of Cell-293T. In addition, we also proposed a physical model for the dispersion of Mg-ACC in CS matrix, which could better explain the enhancement of the mechanical properties of nanocomposite films. The above results provide a comprehensive understanding of the development of high-performance bionic nanocomposites loaded with high nanofillers.