Magnetic field distribution in the flare productive active region NOAA 10720

► We examine association of solar flares and magnetic flux in active region 10720. ► We calculate the normal magnetic field distributions from line-of-sight field. ► It is shown that X class flares appear, when the magnetic flux reaches ∼1022Mx. ► During the flare X3.8 no considerable change of the magnetic flux is observed. ► The idea of flare energy accumulation in the coronal current sheet is supported.