The　low-frequency　instability　in　variable　speed　constant　frequency　doubly　fed　induction　generator　(DFIG)　widely　used　in　wind　power　generation　is　investigated.　The　low-frequency　oscillation　phenomena　in　DFIG　are　analyzed　via　numerical　simulation　as　the　input　mechanical　torque　increases.　The　simulation　results　show　that　the　low-frequency　oscillation　enhances　total　harmonic　distortion　(THD)　of　stator　current,　thus　decreases　the　system　performance.　Then　a　mathematical　model　of　the　closed-loop　system　is　derived.　The　Jacobian　eigenvalue　analysis　shows　that　the　low-frequency　oscillation　is　essentially　caused　by　the　Hopf　bifurcation,　thus　the　stability　boundaries　are　given　according　to　this　analysis.　This　study　is　helpful　to　better　understand　nonlinear　dynamics　of　DFIG,　and　is　valuable　to　parameter　design　for　guaranteeing　stable　operation　of　this　system.