To enhance the ability of doubly fed induction generator（DFIG） and its driving control system to be immune to power grid voltage dip， and to realize the low voltage ride-through（LVRT） function of DFIG-based wind turbines， this paper studies deeply the essence， characteristics and the relationship with DFIG operation state of the electric-magnetic transition， on the basis of the algebraic equations of DFIG. The transition， initiated by voltage dip， is researched quantitatively and qualitatively. Through the quantitative research， the rules of the electro-magnetic variation and the effect of the operation state of DFIG during the transition is shown clearly; and through the qualitative research， the property and the mechanism of the electro-magnetic transition are further discovered.