The invention discloses a preparation method of a special coupling agent for ultrasonic flaw detection on a rough surface, belonging to the technical field of preparation of coupling agents. The preparation method comprises the following steps: firstly, soaking wheat straws in biogas broth to initially degrade and activate wheat straw fibers; secondly, cutting the straws, soaking the straws in lime water, digesting the straws, then putting the straws in a stone mortar, beating the straws, then putting short straws in a sodium hydroxide solution, and dissolving lignin in the straw fibers by using microwave heating combined with high-pressure heating treatment; and finally, mixing filter cakes with chloroacetic acid, performing centrifugal separation after heating reaction, and drying a precipitate into powder, thereby obtaining the special coupling agent for ultrasonic flaw detection on the rough surface. According to the preparation method provided by the invention, the wheat straws, which are low in price and easy to obtain, are used as a raw material, the obtained product only needs to be confected to be with different viscosities for direct use, the product is green and nontoxic, the surface of a workpiece can be washed directly with water after completion of flaw detection, and soil and water are not polluted. By use of the coupling agent, the problems that a conventional coupling agent for ultrasonic flaw detection on the rough surface pollutes a workpiece after use and is difficult to clean and recover are effectively solved.