Dear Chenxu.Zhao,
Indeed your simulation is suffering the same edgewise / side-to-side instability discussed earlier in this forum topic. For the NREL 5-MW wind turbine, the instability will be present in parked/idling simulations in high winds with yaw errors between 20-40 degrees. The frequency you are reporting is the frequency of the blade-edgewise modes. Disabling the nacelle-yaw DOF (YawDOF) will not solve the problem, but disabling the edgewise DOF (EdgeDOF=False) or increasing the edgewise damping (BldEdDmp(1)) would. See the discussion above for further guidance.
Best regards,