The invention relates to a dextran nano-drug carrier, a preparation method thereof, anti-tumor drug and a preparation method thereof. The dextran nano-drug carrier comprises carboxylated dextran nano particles; and grain sizes of the carboxylated dextran nano particles are 20-100 nanometers. The carboxylated dextran nano particles with the grain sizes of 20-100 nanometers have good biological inclusivity and degradability, and can load drugs to act on human bodies. Moreover, owing to structural features of carboxylated dextran, the drugs can be loaded through hydrazone bonds, and the hydrazone bonds are sensitive to pH (potential of hydrogen), and break in a specific pH environment. Therefore, the dextran nano-drug carrier can be used for establishing a pH responsive drug releasing system, so that a curative effect is improved, and side effects are reduced.