The present invention relates to an oligonucleotide structure and a method for preparing the same and, more particularly, to an oligonucleotide structure in which a polymer compound is linked to an oligonucleotide via a covalent bond to improve in vivo stability of the oligonucleotide and cellular delivery efficiency of the oligonucleotide; and to a method for preparing the same. The oligonucleotide structure is improved into a homogenous material, thereby solving the problem in material verification due to polydispersion characteristics occurring when a hydrophilic material linked to the oligonucleotide is a synthetic polymer; the oligonucleotide structure is easy to synthesize compared with the existing process; and the size of a double-stranded oligo RNA structure can be accurately adjusted through the control of the repetition number of a hydrophilic material block, and thus, the gene expression regulation function of the oligonucleotide does not deteriorate through the synthesis of the optimized oligonucleotide structure, and the oligonucleotide can be delivered into cells at even a relatively low-concentration dosage. Therefore, the oligonucleotide structure of the present invention can be useful as a novel type oligonucleotide delivery system as well as a tool for treating cancers, infectious diseases, and the like.