The new advances in the researches on the core-mantle boundary (CMB) dynamics are summarized in this paper. They are the studies of the structure and characteristics of the CMB, the topography of the CMB, the patterns of the mantle convection near the CMB and the core flow at the top of the outer core. On the basis of the coupling between the core and mantle, the decadal fluctuation in the Earth' s rotation is discussed. We also introduce the basic theory of the electromagnetic coupling, the viscous coupling and the topography coupling at the core-mantle interface and their influences to the decadal fluctuation in the Earth' s rotation. At the end of this paper authors emphasize that in order to understand the nature of the decadal fluctuation in the Earth' s rotation the best way is to develop a coordinate model, which will joint different scientific areas such as the astronomy, geophysics, geodynamics, space geodecy and so on.