The invention relates to the technical field of soil heavy metal remediation, and specifically discloses a method for preparing an iron-based bio-char material, a prepared iron-based bio-char material, and a method for applying the iron-based bio-char material in treating soil heavy metal pollution. According to the material, biomass is adopted as a raw material; a high-temperature carbonization method is adopted; during the bio-char preparation process, an iron-containing compound is added, such that iron is doped according to a certain ratio, and the iron-based bio-char material with special structure and function is formed. The material has the advantages of simple preparation process, low production cost, and short production period. The obtained iron-based bio-char material has a unique effect in repairing arsenic-cadmium composite polluted soil. With the material, bio-availability of arsenic and cadmium in soil can be effectively reduced, arsenic and cadmium contents in agricultural products planted in the arsenic-cadmium composite polluted soil can be greatly reduced, and no toxic or side effect is caused on crops. The material is safe to apply, and can be used in a large scale in treatment of arsenic-cadmium composite polluted soil.