Abstract: The theory and method for calculating the force of vertical cables by a frequency-based method were analyzed. The difference between a practical cable and the model of a theoretical uniform cable and the significance of the extreme properties for calculating the cable force of vertical cables were pointed out. According to the structural characteristic of a practical cable, the vibration model consisting three segments and the rigid constraints at the two ends was proposed, which could obtain the practical proportion variable by modifying the uniform proportion variable using the formulas of calculating the cable force with the uniform properties. With the finite element method, the parameter influence of extreme properties were analyzed, and the changing regularization of the proportion variable amending coefficient (PVAC) against the length amending coefficient (LAC) and the stiffness amending coefficient (SAC) was researched. The following results were drawn: i) The PVAC was directly proportional to the LAC, and the slope of the functional equation was SAC; ii) The SAC increased with the stiffness ratio and decreased with cable force respectively. Based on above results, the functional equation of the SAC was constructed and the amending formulas of calculating cable force were created finally, which could effectively reckon in the extreme properties affections. The results of numerical analysis and engineering examples showed that a) the effect of the extreme properties on the calculation of cable force rapidly increased with respect to the length ratio when the length ratio was bigger than 0.08, b) the uniform formulas of calculating cable force cannot obtain a correct cable force when the length ratio was bigger than 0.14, and the calculating error could be over than 50%; c) the amending formulas of calculating cable force proposed could throughout obtain a correct cable force when the length ratio was less than 0.5, and the calculation accuracy satisfied the engineering requirements.