We report the observation and coronal magnetic field models of two successive flares, the confined M1.0 (SOL2012-03-06T22:49) and the eruptive X5.4 flare (SOL2012-03-07T00:02) of NOAA active region 11429, in order to understand their magnetic cause and back-reaction. The observation by the Atmospheric Image Assembly (AIA) on the Solar Dynamics Observatory (SDO) reveals small-scale brightenings prior to both flares, along the major flaring polarity inversion line (PIL). The locations of precursor brightenings on AIA images spatially coincide with highly sheared regions and footpoints of near-surface loops, suggestive of a tether-cutting like reconnection before the flare. Both the M and X flare initiated at the same part of the PIL with similar 3-D magnetic field topologies. The newly released 135-second cadence magnetic field data from Helioseismic and Magnetic Imager (HMI) onboard SDO were used as the boundary conditions, from which both three-dimensional (3D) nonlinear force-free (NLFF) and non-force-free (NFF) field models were extrapolated. With these models, we analyzed the coronal magnetic field evolution and found the evidence of back-reaction associated with coronal magnetic field restructuring.