Because the stator winding is directly connected with the power grid, the Doubly Fed Induction Generator (DFIG) is sensitive to the fault of the power network. Among all the grid fault types，the unbalanced fault is the highest frequency of the fault. A novel co-simulation method has been presented in this paper. Then 1.5MW DFIG wind power system has been built via a coupled simulation:Ansoft Maxell, Matlab/Simulink and Ansoft Simplorer. The co-simulation can not only consider the real factors of the motor model, but also fully realize the simulation of doubly fed induction generator system. and the variation of DFIG under single-phase-to-ground dip of type B is simulated， the changes of the stator flux and the rotor voltage of the DFIG under single-phase-to-ground dip of type B is obtained. Finally, the simulation results are in agreement with the theoretical analysis, which verifies the validity of the transient process.