Abstract: Hydrodynamic coupler is used for startup tool in the large inertia mechanical equipment. The incomplete neutrality of impeller installation and loading perturbation cause the input shaft of pump wheel to vibrate. Internal flow characteristics of hydrodynamic coupler are affected by the vibration of the pump wheel. And the external performance of hydrodynamic coupler is determined by its distribution of internal flow field. Therefore, it is very important to make a deep research on the distribution of internal flow field under the condition of vibration. Numerical simulation is a main way to study the internal flow field of hydrodynamic coupler. The simulation physical model was created firstly by using the software of ICEM (integrated computer engineering and manufacturing), and hexahedron and tetrahedron cells were used to partition the calculation region to generate the grids. The hexahedron was used in main channel of pump wheel and turbine. The tetrahedron was used in boundary motion region. And then the software of FLUENT was used to perform the simulation. The UDF (user-defined function) of FLUENT was used to define the parameters of dynamic mesh control, as well as the axial velocity of pump. Realizable k-ε model was used, besides, the turbulence model and the second-order upwind scheme were adopted for solving the momentum and kinetic energy equation, and the PISO (pressure-implicit with splitting of operators) algorithm was used for pressure and velocity coupling. With the pump axial moving, the boundary of the corresponding flow field would change. The dynamic mesh model was used for boundary motion domain caused by vibration. The results of numerical simulation that are calculated by different two-phase flow models were quite different. In order to obtain accurate and reliable results of numerical simulation, the numerical simulation and external characteristic experimental results were compared. It showed that the error of VOF (volume of fluid) model was less than 5%, and the error of Mixture model was over 20%. It showed that the simulation results by VOF models were more accurate and close to the experimental results. Furthermore, the external characteristics and phase distribution law of fluid coupling were also compared and analyzed under different axial vibration status. And the results indicated that the vibration of the pump wheel could make the flow pulsation increase. Under the condition of radial vibration, the disturbance direction was perpendicular to the gas-liquid interface. A larger wave crest could be formed within pump wheel. However, due to the centrifugal force in the pump wheel, the wave would rapidly decrease. Therefore, the flow pulsation in the turbine was relatively small, that was to say, the torque change was relatively small. Under the condition of axial vibration, the direction of vibration was the same or opposite to the direction of circulation. Therefore, it would effectively enhance the fluctuation of the flow pulsation and cause the larger fluctuation of turbine torque. Numerical calculation showed that the higher the rated speed, the larger the torque ripple amplitude of pump turbine and the fluctuation range of radial force and axial force. The vibration period decreased and the deviation of the torque ripple of the pump turbine was bigger. Vibration would lead to the decrease of the transmission torque, and the axial vibration had a greater impact on the transmission torque, and a smaller influence on the radial force. The vibration would cause the pump wheel and turbine torque to fluctuate, and the pulsation amplitude increased with the increase of the vibration amplitude. When the amplitude of vibration was less than 0.02 mm, the amplitude of torque was smaller. But when the amplitude of vibration was 0.04 mm, the amplitude of torque increased sharply. On that basis, the axial clearance value should not be more than 0.04 mm (the axial clearance was twice of the amplitude of vibration).