The increasingly large scale of wind power and low voltage ride through technology is becoming mature, there is an urgent need to know the motor inside the low voltage ride through the conditions of the electromagnetic field changes, to provide the basis for the motor fault diagnosis. Electromagnetic field of doublyfed induction generator (DFIG) during the grid voltage drop to different extent of the motor were analyzed, and on this basis, analysis the different and changing trends of the electromagnetic field inside the motor, was used as the basis for the doubly fed wind turbine fault diagnosis.