Doubly-Fed Induction Generator (DFIG) often adopts Crowbar protective circuit to realize Low Voltage Ride Through (LVRT) under voltage drop fault of power grid. Crowbar resistance will affect LVRT effect of the unit significantly. Time domain expression of transient current of the wind turbine generator system under the voltage drop fault and the calculation formula of fault current at the rotor side are deduced through space vector analysis and Laplace transformation based on the transient mathematical model of DFIG system under voltage drop. A Crowbar resistance adjustment method conforming to engineering practices is proposed, which overcomes the overcurrent at the rotor side and overvoltage of DC bus after Crowbar protective circuit is applied. Both case study and the simulation test demonstrate that the proposed method can inhibit the transient fault component effectively and increase the LVRT level of DFIG system significantly. This work is supported by Natural Science Foundation of Hunan Province (No. 2017JJ4024).