The invention discloses a method for preparing a coal gangue magnetic adsorbent by a two-step method, which belongs to the field of resource circulation and environmental protection, and is characterized by comprising the following steps: crushing and screening dry coal gangue to 200-325 meshes, mixing with soluble ferric iron salt according to a mass ratio of 20: 1-4: 1, adding water, stirring, drying, and calcining and activating for 2-4 hours at 400-600 DEG C in an air atmosphere; mixing the cooled fired product with a soluble ferrous salt, wherein the molar ratio of the ferrous salt to theferric salt in the previous step is 1: 1; and drying the mixture, and calcining the dried mixture in a closed resistance furnace at 400-550 DEG C for 1-2 hours; carrying out grinding, screening, magnetic separation and other steps on the obtained fired product to obtain the magnetic adsorbent. The trivalent and divalent iron salts are respectively used as a catalyst and a magnetizer, and the coalgangue adsorbent with strong magnetism is prepared by two-step calcination, so that the environmental problem caused by repeated addition and use of various catalysts, magnetizers and additives is avoided. The prepared magnetic adsorbent not only has high adsorption efficiency on pollutants such as dyes and phosphorus, but also has good magnetic separation performance and good regeneration effect.