The invention discloses a method for preparing a magnetic flocculant by compounding straws and biochar. The method comprises the following steps: preparing the biochar: crushing, carbonizing, carryingout hydrochloric acid washing and drying on acrous calamus straws; processing the straws: adding sodium hydroxide into the acrous calamus straws for modifying and drying by a grinding agent; mixing the biochar with the modified straws according to the ratio of 1 to 1 and then adding powder sodium carbonate for mixing to generate biochar composite straws; preparing magnetic hydrotalcite and addingbiochar composite straw powder according to the ratio of 1 to 1, thus obtaining the magnetic flocculant. The superiority of the magnetic flocculant disclosed by the invention is that the straws as the raw material are natural high polymer materials; the flocculant prepared by combining the cellulose, lignin, the biochar and the magnetic hydrotalcite can effectively remove suspended particles andhydrophilic substances in water, and is recycled; the flocculant subjected to magnetize treatment has the advantages of capability of being reused for multiple times, low cost and good effect.