The invention relates to a method for modifying a small-span concrete aqueduct into a large-span cable-stayed structure, and belongs to an old aqueduct structure modifying method. A modern new structure and a new material technology is utilized to modify an old structure, construction is completed on the premise that normal operation of an original project is not affected, the small-span aqueduct is modified into the large-span cable-stayed structure, using functions are effectively improved, and the service life is prolonged. According to a green water conservancy ideal, the functions and the service life of the aqueduct can be recovered at a low cost, and the purposes of being low in carbon, environment-friendly, water-saving, waste-using, resource-saving and low in building garbage are achieved, so that the method is a green invention for the water conservancy industry.