Abstract: Provided are an attenuated virus of a flavivirus virus and the use thereof. The attenuated virus comprises a polyadenylic acid (poly(A)) sequence, wherein the polyadenylic acid (poly(A)) is used for replacing a part of the nucleotide sequence of a 3? untranslated region (3?UTR) of the flavivirus virus, so that the 3? untranslated region (3?UTR) of the attenuated virus obtained after the part of the nucleotide sequence of the flavivirus virus is replaced at least retains a 3?-end stem loop region (3?SL). The attenuated virus can be used for preparing safe and effective attenuated vaccine strains.