The invention discloses a preparation method of modified blue-green algae biochar, wherein the preparation method comprises the steps: S1, washing blue-green alga cleanly, carrying out freeze drying,grinding, pulverizing the blue-green algae, and carrying out carbonization treatment in an inert atmosphere; grinding and sieving the blue-green algae powder, soaking the blue-green algae powder in anacid solution, washing with water to be neutral, and drying to obtain blue-green algae biochar; and S2, dissolving an iron-containing compound and the blue-green algae biochar in an ethanol aqueous solution, placing the solution on a stirrer, uniformly stirring and mixing, dropwise adding a reducing agent in an inert atmosphere, continuously stirring after dropwise adding is finished, carrying out centrifugal separation on the obtained mixed solution to remove supernate, and washing and drying the obtained solid to obtain the modified blue-green algae biochar. On one hand, the blue-green algae waste is used as the raw material, the raw material is rich and easy to obtain, and a new thought is provided for resource utilization of the blue-green algae waste; on the other hand, the preparedblue-green algae biochar loaded nano-iron is uniformly dispersed, the problems that blue-green algae is difficult to treat and utilize, nano-particles are prone to agglomeration and passivation and the like are effectively solved, and the arsenic adsorption and removal effect of the material is improved.