Vibration based cable force identification methods have been widely used in the field of bridge health monitoring and inspection for their simple and effective properties. However， the inclination angle on the cable force identification accuracy is not considered as horizontal arranged cable model is adopted in the theory derivation. In fact， the angle of inclination leads to the linear distribution of cable force along the length direction， and the angle of inclination will have an impact on the sag of the cable， which affects the accuracy of cable force identification. In this paper， the modal parameters of the cables fixed at both ends are calculated by the finite difference method and the corresponding cable forces are identified by iteration method. Then the influences of the inclination angle on the modal parameters of the cable and the accuracy of the identified cable force identification are investigated via numerical example. The results show that the force identification errors are very large when the first order frequency is used for small-sag cables and the first or second order frequency is used for large-sag cables without considering the inclination effects.