The invention discloses a composite modified biochar preparation method, which comprises: (a) preparation of wheat straw biochar: carrying out air-drying on wheat straw, crushing, sieving, carbonizing, cooling to a room temperature, washing with an acid, filtering, washing to achieve a neutral state, and drying to achieve constant weight; (b) preparation of alkali-modified biochar: soaking the wheat straw biochar in an alkaline solution, carrying out heating stirring, naturally cooling, filtering, washing with distilled water, and drying; and (c) preparation of magnetic / alkali composite modified biochar: dipping the alkali modified biochar in an iron ion solution, adjusting the pH value, carrying out mixing standing, carrying out ultrasonic wave treatment, carrying out heating stirring, drying, and washing to obtain the magnetic / alkali composite modified biochar. According to the present invention, the advantages of the two modification technologies are combined by adopting the composite modification method, such that the adsorption efficiency on cadmium in wastewater is improved.