Abstract: Managed pressure drilling (MPD) is an important technique to solve narrow drilling mud density window problem in deepwater drilling. The top end of the MPD riser is a rotating control device (RCD), and the variation of its vertical position will directly affect the bottom hole pressure. Currently, literatures regarding the prediction method of vertical position of the RCD could be found sporadically. In this work, based on Euler-Bernoulli beam theory, considering the influence of high pressure inside the riser and drilling fluid flow rate, the dynamic theoretical model of MPD riser system was established by using direct stiffness matrix method, and Newmark-β integral method was employed to solve the dynamic equations of the MPD riser system. Further, dynamic prediction of the vertical position of the RCD under random waves and vessel motion was researched. Taking a 1600 m deepwater riser in the South China Sea for example, the calculation showed that under one year return sea states, when inside pressure of the riser is 12 MPa, the drilling fluid flow rate is 2 m/s, and the drilling fluid density is 1600 kg/m³, the vertical position of the RCD fluctuated between 0.5 to 0.6 m. However, when the drift motion of vessel was taken into account, the fluctuation of the vertical position of the RCD will be 0.2～0.7 m. Based on this, the fluctuation range of bottom hole pressure could be calculated according, thus providing a reference for precise control of bottom hole pressure during managed pressure drilling operations.