The invention discloses a magnetic biological carrier and a preparation method thereof. The raw materials of the magnetic biological carrier comprise aggregate, iron oxide, a binder and a reducing agent. The preparation method of the magnetic biological carrier comprises the following steps of: mixing the aggregate, iron oxide and the reducing agent, crushing, screening with a 60-mesh sieve to obtain a mixed material, adding water which is 30-60% of the mixed material by weight to the binder, stirring for dissolving the binder to obtain a binder solution, then mixing the binder solution with the mixed material, uniformly stirring, granulating and shaping to obtain 2-10 mm granules, drying the granules till the water content of the granules is less than 10%, and calcining at the temperature of 350-1100 DEG C for 0.2-10 hours to obtain the magnetic biological carrier. The magnetic biological carrier provided by the invention can be applied to the deep treatment of secondary treatment water of domestic sewage, meets the requirements of landscape water replenishing and circulating cooling water treatment, and can be used for realizing the deep treatment and resource utilization of industrial wastewater.