In this study, a 3D idealized model of tidal flow, in which the tidal elevation and velocities are solved analytically, is developed. The horizontal eddy viscosity is neglected, and the vertical eddy viscosity used in the study is assumed to be independent of time and only varies as a parabolic function in the vertical direction. The analytical solution is obtained in a narrow rectangular bay, with the topography varying only across the bay. The model results are compared with the field observations in the Xiangshan Bay. The results show that the influence of varying vertical eddy viscosity mainly has two aspects. On one hand, it amplifies the magnitude of the tidal elevation, particularly the amplitude near the head of the bay. On the other hand, it adjusts the axial velocity profile, resulting in an obvious frictional effect. Furthermore, the tidal elevation and velocities are more sensitive to the magnitude of the eddy viscosity near the bottom than the structure in the upper water layer.