The invention discloses a method and a system for preparing a magnetic biochar composite material. The method includes the steps: (1) performing pyrolysis treatment on biomass materials to obtain high-temperature oil gas and biochar; (2) activating the biochar by water vapor to obtain activated biochar; (3) crushing the activated biochar to obtain activated biochar powder; (4) preparing mixed solution containing ferric iron salt and ferrous iron salt and impregnating the activated biochar powder in the mixed solution; (5) adding alkali liquor into mixture obtained in the step (4), performing precipitation reaction to generate ferroferric oxide and loading the ferroferric oxide in the activated biochar powder; (6) filtering, cleaning and drying a product obtained in the step (5) to obtain the magnetic biochar composite material. The magnetic biochar composite material prepared by the method has good stability and magnetism and strong adsorption capacity.