The cable tension test by frequency method is widely used in the construction control, assessment and long-term monitoring of cable-supported structures, because of the existence of intermediate supports such as absorber and damper will reduce the calculation precision of cable tension. In order to acquire a cable tension calculation method which can consider the influence of intermediate supports, the paper proposed an equivalent cable length method based on cable vibration differential equation and energy principle. All the practical formulas considering fixed-fixed boundary condition of cables can be used reasonable by using the method. The results from case studies demonstrate that the method can simulate the tension condition precisely for cables with intermediate supports. Meanwhile, as the formula of equivalent cable length is not complex, so it can be easily used in practical engineering applications.