Pressure fluctuation in the flow field around a tidal current turbine is one of the key factors affecting the safe and stable operation of the turbine. To evaluate the effects of the duct and the duct-to-rotor clearance ratio δ on the pressure fluctuation characteristics of the tidal current turbine, three-dimensional transient CFD methods and slip-grid techniques were used to perform three-dimensional numerical simulations of the bare turbine and the ducted turbine with the blade tip clearance of δ=0.02D and δ=0.06D, respectively, under the optimal operating conditions (T_TSR=4). By analyzing the unsteady flow phenomenon of the flow field around the turbine, the pressure pulsation and frequency-domain vibration characteristics of the turbine under three different working conditions were obtained. The results show that the pressure fluctuation amplitude of the turbine is basically increasing from the root to the tip of the blade; The addition of a duct can effectively reduce the pressure fluctuation amplitude of the turbine; δ has less impact on the pressure fluctuation coefficient of the turbine and more impact on the vibration characteristics in the frequency domain of the pressure fluctuation; The main frequency of the pressure fluctuation of the turbine is mainly concentrated near the leaf frequency of the corresponding operating conditions. In addition, it is also found that the pressure fluctuation of turbine blade suction surface is stronger than the pressure surface, and the addition of duct can effectively reduce this fluctuation.