The present invention discloses a large-molecular hydrophobic anhydride modified starch and preparation method thereof. The method includes drying the starch until the starch has moisture mass content of between 8 and 13%; spraying the dilute alkali solution into the resultant starch to be mixed and dried so that the starch has moisture mass content of between 8 and 13%; adding the liquid maleic anhydride modified polyethylene or the solid maleic anhydride modified polyethylene dissolved and diluted by the acetone, heating and stirring in the kneader to fully and evenly mix the starch with the hydrophobic acid anhydride; moving the mixture into the sealed reaction vessel for reaction for 1 to 3 hours at the temperature of between 100 and 130 DEG C, cooling the resultant product to room temperature to obtain the large-molecular hydrophobic modified starch. According to the invention, the gelatinization temperature of the resultant product is decreased and the viscosity stability is improved, solving the compatible problem of the starch with the polyethylene, so that the tensile strength of Zemac modified starch extruded molding compounds are significantly increased, having good prospect of application in starch based biodegradable material.