The invention discloses a high-yield method for preparing a ribonucleic acid (RNA)-diethyltriamine pentaacetic acid (DTPA) conjugate. The conjugate is prepared by conjugating a RNA or locked nucleic acid (LNA) having an amino group to p-isothiocyanate benzyl-DTPA (SCN-Bn-DTPA) or DTPA anhydride with heating under microwave.