An oligomer-contained nanoparticle complex is provided. The oligomer-contained nanoparticle complex of the invention comprises (a) a nanoparticle, (b) a polymer with high molecule weigh, (c) a target molecule, (d) an oligomer, wherein the oligomer is crosslinked with the polymer by the intermolecular hydrogen bonds or electron bonds, and e) a space for active substances, wherein the space for active substances is encapsulated by the nanoparticle. The oligomer-contained nanoparticle complex of the invention has the high stability and fast drug release rate, and would not be largely accumulated in normal tissues. The invention also provides a method for preparing the oligomer-contained nanoparticle complex and the method for releasing drug.