The invention relates to a preparation method of a favipiravir impurity 6-chloro-3-hydroxy-hydroxypyrazine-2-formamide shown as formula (1). By preparing the favipiravir impurity, and providing a reference substance for qualitative and quantitative analysis of favipiravir impurity, the invention improves the quality standard of favipiravir, and provides important guidance for safe medication of favipiravir.