For purpose of a more comprehensive understanding of the flow in the disk cavities, the sealing flow in both the upstream and downstream disk-cavity system of a 1.5 stage turbine is investigated by means of CFD methods. The result indicates that the flow structures in the cavity of the unsteady results are less stable than that of the steady results. The comparison of the upstream and downstream results shows that the downstream sealing flow generally is the mirror image of the upstream sealing flow, while the downstream rim seal performs better as the main flow works on the rotor blades. In addition, the proportions of eigenvalues of the first POD mode demonstrates strong unsteady characters of the flow in the rim seal. The frequency spectra of the POD time coefficient indicate that the most prominent frequency of the flow field in the rim seal is caused by the rotor blade.