  Magnetic flux rope (MFR) plays an important role in solar activities. A
quantitative assessment of the topology of an MFR and its evolution is crucial
for a better understanding of the relationship between the MFR and the
associated activities. In this paper, we investigate the magnetic field of
active region 12017 from 2014 March 28 to 29, where 12 flares were triggered by
the intermittent eruptions of a filament (either successful or confined). Using
the vector magnetic field data from the Helioseismic and Magnetic Imager on
board the \textit{Solar Dynamics Observatory}, we calculate the magnetic energy
and helicity injection in the active region, and extrapolate the 3D magnetic
field with a nonlinear force-free field model. From the extrapolations, we find
an MFR that is cospatial with the filament. We further determine the
configuration of this MFR by a closed quasi-separatrix layer (QSL) around it.
Then, we calculate the twist number and the magnetic helicity for the field
lines composing the MFR. The results show that the closed QSL structure
surrounding the MFR gets smaller as a consequence of the flare occurrence. We
also find that the flares in our sample are mainly triggered by kink
instability. Moreover, the twist number varies more sensitively than other
parameters to the occurrence of flares.
