The invention provides a method for preparing a nano granular medicinal composition. The method comprises the following steps of: making a water-soluble medicament and a carrier protein-induced gold nano cluster in contact with a hydrophilic polymer in water to obtain a contact product, and centrifuging and separating the solid from the contact product. Through the contact condition, the hydrophilic polymer can encapsulate the water-soluble medicament and the carrier protein-induced gold nano cluster to form nano granules with the diameters of 100 to 300 nanometers; and in the hydrophilic polymer, the total content of an acrylic structural unit and an N-isopropyl acrylamide structural unit is at least 90 weight percent. The invention also provides the nano granular medicinal composition prepared by the method. The nano granular medicinal composition prepared by the preparation method can be controlled to release by controlling the pH value and the temperature, and in-vivo positioning of the composition can be observed by fluorescent imaging.