Based on a more perfect finite element model of stay cable, the relative sensitivity of calculational frequencies for long stay cable and short stay cable were analyzed respectively, according to the different design variable of cable such as tension force, tensile rigidity, flexural rigidity , density and the most sensitivity design variable was established. On the basis, according to the optimization theory, the dynamic finite element model updating of a stay-cable of Dongting Lake Bridge based on the most sensitive design variable was processed. The final updated FE model was able to produce natural frequencies closer to the test ones. The updated FE model was useful for the reference model to tension force assessment.