The present invention relates to a composition for detecting nucleic acid comprising duplex molecular beacon and graphene oxide and a colorimetric signal enhancement method of detecting nucleic acid using the same. According to the composition, kit and method for detecting nucleic acid of the present invention, a complex can be formed by adsorbing a single strand having a DNAzyme sequence dissociated from the conjugate of a duplex molecular beacon and a target nucleic acid to the graphene oxide surface, and separated, and a colorimetric signal amplified therefrom can be induced, so that a very low concentration of target nucleic acid can be detected with high efficiency and the target nucleic acid can be detected quickly and easily in seconds. Therefore, a new colorimetric target nucleic acid detection system capable of point of care testing (POCT) can be provided.