The invention discloses a preparation method and an application of a catalyst for efficient cracking of biomass tar. According to the catalyst, cheap biomass waste is taken as a precursor, nano-iron is taken as an active center, and with the adoption of a one-step hydrothermal carbonization synthesis technology, iron ions are reduced into zero-valent iron and chelated on the surface of hydrothermal carbon. The hydrothermal carbon based catalyst has good catalytic degradation effect on tar pollutants such as phenol and the like, can realize stable occurrence in an acid-base system, does not produce secondary pollution and is an environment-friendly type carbon based functional nanomaterial, and the preparation process is simple.