Aimed at the problem of main cable form-finding of suspension bridges in the finished dead load state and the unloaded state， a modified force density method is proposed. This method combines the concept of unstressed length， assuming the initial cable shape is straight line segment. A structural balance equation is established to find the target cable shape under limited conditions by iterations， which is based on the force density method. The proposed method was used to calculate the cable shape of the Beijing-Hangzhou Grand Canal Suspension Bridge by Matlab. The comparison of the numerical calculation results and measured values of the coordinates shows that the proposed method has a stable convergence， high computational efficiency and high accuracy. The proposed method was verified to be suitable for self-anchored suspension bridges with three-dimensionally cables through the calculation example.