  We investigate the three-dimensional (3D) magnetic structure of a blowout jet
originated in the west edge of NOAA Active Region (AR) 11513 on 02 July 2012 by
means of recently developed forced field extrapolation (FFE) model. The results
show that the blowout jet was caused by the eruption of the magnetic flux rope
(MFR) consisting of twisted field lines. We further calculate the twist number
$\mathcal{T}_{w}$ and squashing factor Q of the reconstructed magnetic field
and find that (1) the MFR corresponds well to the high $\mathcal{T}_{w}$ region
(2) the MFR outer boundary corresponds well to the high Q region, probably
interpreting the bright structure at the base of the jet. The twist number of
the MFR is estimated to be $\mathcal{T}_{w}=-1.54\pm 0.67$. Thus, the kink
instability is regarded as the initiation mechanism of the blowout jet as
$\mathcal{T}_{w}$ reaching or even exceeding the threshold value of the kink
instability. Our results also indicate that the bright point at the decaying
phase is actually comprised of some small loops that are heated by the
reconnection occurred above. In summary, the blowout jet is mostly consistent
with the scenario proposed by \citet{mcs10} except that the kink instability is
found to be a possible trigger.
