The invention provides a preparation method of straw-based magnetic charcoal. The method includes the following steps that (1) straw is cleaned, naturally air-dried, dried in a drying oven, smashed and sieved, and straw powder is obtained; (2) the straw powder is pre-carbonized in a muffle furnace, washed and dried, and a straw-based pre-carbonized product is obtained; (3) the straw-based pre-carbonized product is put in a tube furnace, argon is fed, pyrolysis is performed at the temperature of 400-500 DEG C for 2-3 hours, cooling is performed till the temperature reaches the room temperature,washing is performed till the product is neutral, drying and grinding are performed, then sieving is performed, and straw-based charcoal is obtained; and (4) the straw-based charcoal and oleylamine are put in a reaction kettle for uniform mixing and ultrasound treatment, ferric acetylacetonate is added into a mixed solution, the mixed solution is stirred uniformly, a liquid product is obtained, ablack substance is separated out after centrifugation is finished, and the straw-based magnetic charcoal is obtained. Compared with original charcoal, the prepared straw-based magnetic charcoal has the advantages of being better in thermal stability and larger in specific surface area, and medium and large pores in the pore structure of the straw-based magnetic charcoal prevail.