The invention relates to a design of an isodirectionally turning stay cable system of a cable-stayed bridge. The design comprises the following specific design operation steps that: establishing the geometry and mechanical equation of a stay cable system, carrying out multiple circulation correction calculation on the system until an accurate parameter U to be determined is obtained through a known parameter A; establishing a descriptive geometric equation of the outer surface of a cable tower, and calculating an accurate position of an exit point of a saddle on a tower; establishing the universal databases C, S and B of the stay cable, the saddle and an anchor tensile plate, and carrying out construction piece size design and material amount calculation; carrying out simulation lofting, checking conflicts, and correcting the design; and gathering design parameters, and carrying out material amount statistics. Compared with the prior art, the invention puts forward the method for carrying out the accurate design of the system on the isodirectionally turning stay cable system, the problems of new technology and difficulty in popularization of the stay cable system are solved, and the practical development and the large-scale application of the original technology of the isodirectionally turning stay cable of the cable-stayed bridge are prompted.