Abstract: To find the form of suspension cables precisely, the Newton-Raphson iteration is used and the zero order optimization arithmetic is also dedicated to control the procedure. Meanwhile, the factors that influence the precision of solution are analyzed. Based on this, the form-finding of Runyang suspension bridge under dead load is taken as an example, and the results are compared with those of membrane theory and data from field test. The results show that through the control of the coordinate of the predefined points, the more precise solution can be achieved. This method can be used in form-finding and construction control on main cables of suspension bridge and other long-span cable structures.