Doubly-fed induction generators (DFIG) has different transient characteristics under different generator terminals voltage dip levels. The analysis of the fault transient characteristics is very important to the grid security and stability against the background of large-scale wind generation on power grid. This paper takes asymmetrical voltage sag occurred at the terminal as an example. Based on the rotor voltage equations, the doubly-fed wind power generator fault current is analyzed under different generators terminal voltage drop levels in unified method. In the case of severe voltage drops in generator terminals, the transient characteristics are analyzed considering the crowbar protection insertion. If the terminal voltage drops mildly, the solution of limiting and calculating the short-current has to consider the influence of the control system and the stator current dynamic process is analyzed qualitatively based on the control system. Finally the paper further analyses stator current variation under different terminals voltage dip levels and validates the short-circuit expressions by simulations.