The invention discloses a modified magnetic sludge biological carbon and a preparation method and applications thereof. The modified magnetic sludge biological carbon is prepared by carbonizing residual sludge, modifying sludge by alkali, and modifying sludge by acid. The provided modified magnetic sludge biological carbon has the advantages of good magnetic property, large specific surface and pore volume, abundant pore structure, many surface adsorption functional groups and adsorption sites, and strong performance on removing target pollutants; and the preparation method has the advantages of simple preparation, easy operation, low cost, high production efficiency, and convenience for large scale production. The provided modified magnetic sludge biological carbon is used to remove tetracycline hydrochloride in water for the first time, has the advantages of large adsorption amount, strong adsorption performance, high adsorption efficiency, simple and convenient operation, short processing period, low processing cost, and easy separation and recovery, and has a strong performance on removing tetracycline hydrochloride. The maximal balance adsorption amount can reach 253.8 mg / g. The performance of the biological carbon is obviously stronger than that of unmodified magnetic sludge biological carbon.