To determine whether stochastic subspace identification（ SSI） based on the output-only measurements can identify the doublet modes of cyclic symmetric structures accurately,taking a brake disc as an example,modal analysis based on SSI and traditional experiment modal analysis（ EMA） for the brake disc were performed and the corresponding moda Sl parameters were extracted. The results show the maximum error of natural frequency is only 0. 66% and the mode shapes coincide. The SSI based on the output-only measurements can also be applied to the cyclic symmetric structures to identify the modal parameters.