The present invention relates to a pharmaceutical composition for treating or preventing hepatitis B, comprising: at least one oligonucleotide selected from the group consisting an oligonucleotide represented by a nucleic acid sequence of SEQ ID NO: 1, 2, or 6 or a nucleic acid sequence complementary thereto; and an oligonucleotide having at least one chemical modification on the oligonucleotide, as an active ingredient, and a method for screening a therapeutic agent for hepatitis B according to the formation of a G-quadruplex by HBV and a candidate material. The pharmaceutical composition forms a G-quadruplex with HBV and reduces cccDNA (covalently closed circular DNA) and thus can be used in the treatment or prevention of hepatitis B.