In order to study output transmission line relay of the wind farm based on doubly fed induction generator (DFIG), it is necessary to evaluate the short circuit current characteristics of output transmission line. For the symmetric fault of output transmission line, by using the transient principle of the stator and rotor flux of a DFIG, the expressions of the stator transient currents of the DFIG wind generation system are derived under both the far terminal fault and near terminal fault. For the asymmetric fault of output transmission line, by using the symmetrical component method and the simplified equivalent circuit of a DFIG, short circuit current characteristics are analyzed qualitatively. The simulation results demonstrate that short circuit current characteristics of output transmission line of the wind farm based on DFIG are different from that of a normal bulk electric power system.