A simplified method for predicting the deflections of cable-stayed suspension bridges considering live loads is presented. In the proposed method the governing equation of the cable-stayed suspension bridge, based on potential energy method, is derived. A Fourier series is used to describe the defections of the bridge deck. It has been shown from the numerical investigation of two different cable-stayed suspension bridges that the iteration procedure has a very high convergence rate. Only a few iterations are needed and a small number of series terms are sufficient for achieving the solutions of the problems. The predictions of the proposed method show good agreement with finite element results.