  We propose a catastrophic magnetospheric model for magnetar precursors and
their successive giant flares. Axisymmetric models of the magnetosphere, which
contain both a helically twisted flux rope and a current sheet, are established
based on force-free field configurations. In this model, the helically twisted
flux rope would lose its equilibrium and erupt abruptly in response to the slow
and quasi-static variations at the ultra-strongly magnetized neutron star's
surface. In a previous model without current sheets, only one critical point
exists in the flux rope equilibrium curve. New features show up in the
equilibrium curves for the flux rope when current sheets appear in the
magnetosphere. The causal connection between the precursor and the giant flare,
as well as the temporary re-entry of the quiescent state between the precursor
and the giant flare, can be naturally explained. Magnetic energy would be
released during the catastrophic state transitions. The detailed energetics of
the model are also discussed. The current sheet created by the catastrophic
loss of equilibrium of the flux rope provides an ideal place for magnetic
reconnection. We point out the importance of magnetic reconnection for further
enhancement of the energy release during eruptions.
