The invention discloses a rare earth-chitosan oligosaccharide complex, in particular to a binary complex formed by the coordination of rare earth ions and chitosan oligosaccharide; the complex is nanoparticles, the average particle size of the particles is 50-100nm, and the particles are distributed evenly. The final concentration of the complex for prompting the apoptosis of K562 cancer cells is 0.001mu g / mL-10mu g / mL, the inhibition ratio of 1mu g / mL of the complex to K562 cancer cells can reach 47% and the complex can be used to prepare the anti-K562 cancer cell drug. Compared with the prior art, the rare earth-chitosan oligosaccharide complex has good water-solubility, low toxicity and high bioavailability, the antitumor activity of the complex is better than any single ligand, the complex has synergetic effect, good chemical stability and good thermal stability, the complex can be used, stored and transported at room temperature; in addition, the preparation method of the invention has the advantage of simple operation, wide raw material sources, green and environmentally friend and the like.