Accurate cable tension identification is the key to health monitoring and condition assessment of cable stayed bridge. Error must not be overlooked when applying vibration based formulas to identify the tension of real cables under sag, stiffness and boundary conditions. By numerically resolving the implicit frequency function of real cables with the above conditions, the indirect cable force identification method and corresponding cable mode frequency calculation method are investigated. The error of tension is studied using numerical simulation, and an empirical error correction formula is presented to fit the relationship between cable force error and cable parameters ξ and λ2. A case study on two real cables form the Shanghai Changjiang bridge shows that the method discussed in this paper can increase the accuracy of the cable force and reduce computing CPU time compared with FEM.