In order to analyze the influence of voltage sag on gear and bearing of the doubly-fed wind turbine drivetrain, rotor finite element method and centralized parameter method were used to carry out dynamic modeling of the disk, shaft, bearing, coupling and gear. Combined with the aerodynamic calculation and control model, the dynamic response simulation program of drivetrain was established. The program was used to simulate the steady-state condition of drivetrain, and the components with the maximum gear meshing force and bearing force were selected. The curve obtained from test of a MW (megawatt) wind turbine were loaded into the dynamic simulation program, the dynamic response of the low voltage sag under various conditions were compared and analyzed. The results show that the voltage sag produces shock loads on the gears and bearings of drivetrain, the shock loads are related to the wind turbine load, voltage sag amplitude and sag form, and the influence factors of these three decreases in turn.