To investigate the influence of annular flow on the dynamic behavior of the rotor, partial differential equations are established to describe the flow in the annulus according to the continuum theory and momentum theorem. The linear perturbation method was used to separate the steadystate part and the dynamic part of the flow field pressure in the annulus. The expression of the dynamic force generated by the annular flow was calculated. A test rig was built to validate the theoretical model. The dynamic model of the dry rotor in the test rig was established using the finite element method. The modal frequency of the rotor tested in the test rig was used to determine the contact stiffness of the flange in the rotor, and then the theoretical model was demarcated. The dynamic force of the annular flow can be expressed by combination of the mass matrix, damping matrix and stiffness matrix. The dry rotor model integrated with the dynamic force becomes the wet rotor model. The variation of the modal frequency and system damping of the wet rotor corresponding to the rotor speed were calculated. The theoretical results were validated by experimental results. The research shows that the annular flow can reduce the modal frequency and can induce the instability of the rotor in some speed domain.