During deepwater riserless drilling operations, the vibration behavior of drill string may have a significant impact on wellbore pressure, which leads to serious drilling accidents such as well leakage and collapse in shallow risk areas. In order to solve this problem, based on Hamilton's principle, a three-dimensional nonlinear coupling dynamics model of drill string in deepwater riserless drilling is established, taking into account the following factors: heave and offset motion of offshore platform, ocean current load, drill string-borehole contact and bit-rock interaction. The Newmark-β method is used to solve the nonlinear discrete equations of the system. The effectiveness of the model and calculation program is verified by the field test data. Meanwhile, a model of wellbore pressure field is established under the influence of axial-lateral-torsional coupling vibration of drill string. The 3D nonlinear coupling vibration characteristic of drill string and its influence on wellbore pressure fluctuation are investigated. The results indicate that the collision between the drill string system and the borehole usually occurs in the middle of the formation and at the bit, which is easy to cause drilling tools failure. The maximum pressure fluctuation along the wellbore is mainly affected by lateral vibration. The mean value of wellbore pressure fluctuation is mainly determined by torsional vibration. The results may be useful to predict the risks of well leakage and collapse in deepwater riserless drilling operations.