In　deepwater　drilling,　accurate　prediction　of　wellbore　gas-liquid　two-phase　flow　behavior　is　significant　for　gas　kick　detection　and　well-control　treatment.　In　this　study,　a　new　transient　gas-liquid　two-phase　flow　model　was　developed　to　simulate　the　wellbore　gas-liquid　two-phase　flow　during　gas　kick　in　deepwater　dual-gradient　drilling　based　on　downhole　separation.　This　model　accounts　for　the　impact　of　a　sudden　change　in　density　on　gas-liquid　flow　behavior.　The　transient　model　was　solved　using　the　finite-difference　method.　The　accuracy　and　stability　of　the　model　were　verified　using　data　measured　from　a　full-scale　experimental　well.　Using　this　model,　the　differences　in　the　apparent　liquid　velocity　and　the　flow　rate　at　the　annular　outlet　during　gas　kick　were　compared　between　deepwater　dual-gradient　drilling　and　conventional　single-gradient　drilling.　Additionally,　the　influences　of　numerous　factors　on　variations　in　flow　rate　at　the　annular　outlet　were　also　studied.　A　new　method　of　early　gas　kick　detection　was　proposed　for　deepwater　dual-gradient　drilling　based　on　downhole　separation.