The present invention provides a nanocomposite hydrogel and a preparation method thereof, and relates to the field of nanocomposite materials. The nanocomposite hydrogel is prepared by mixing completely gelatinized short amylose with an aqueous gelatin solution having a mass concentration of 8%-14%, and then cooling. The present invention utilizes the nanoparticles formed by in-situ self-assembly of the short amylose in the aqueous gelatin solution as a reinforcing agent, and the nanoparticles are uniformly distributed in the hydrogel to form a stable crystallization system, such that the prepared nanocomposite hydrogel exhibits optimal mechanical properties in terms of viscoelasticity, hardness, compressive stress, etc. The preparation process of the present invention is green and environmentally friendly, simple and efficient, and can be widely applied to the fields of food, cosmetics and medicine.