The invention discloses a drug composition taking chitosan oligosaccharide and doxorubicin hydrochloride as active ingredients. A manner of administrating doxorubicin hydrochloride after incubation ofchitosan oligosaccharide and subject cells for a certain time can significantly reduce a concentration of doxorubicin hydrochloride for achieving an equal tumor cell inhibition rate as compared withadministration of doxorubicin hydrochloride alone. The drug composition has a good antitumor effect on MDA-MB-231 human breast cancer cells, PATU-8988 human pancreatic cancer cells and HepG2 human liver cancer cells. A preparation method of the drug composition is simple, and the drug composition is low in cost and easy to apply and popularize in various drug dosage forms.