Abstract: The NREL 5 MW wind turbine with a three-point-supported drivetrain was investigated using SIMPACK multibody dynamics software to examine the axial response of the main bearing under uniform winds of 6-16 m/s and turbulent winds with mean speeds of 10 and 12 m/s, and to assess the influence of tower flexibility. The simulation results show that the hub axial force and power predicted by SIMPACK generally agree with the OpenFAST results and are close to published CFD results at the rated operating condition. At the same mean wind speed, compared with uniform wind, turbulent wind significantly increases the maximum axial velocity and axial sliding ratio of the main bearing. Under the investigated 10 and 12 m/s IEC Class B NTM turbulent conditions and over the 150-600 s statistical interval, the RMS axial relative velocity of the flexible-tower model is about 4.1% and 3.7% higher, respectively, than that of the rigid-tower model, indicating that tower flexibility has only a minor effect on the overall intensity of axial relative motion under the turbulent conditions considered. The results can provide a reference for the analysis of main-bearing axial dynamic response and the setting of support boundary conditions in ground-based drivetrain tests.