Aiming at the instability problem of section coal pillar under the condition of repeated mining in fully mechanized caving face of extra thick coal seam,taking the 18 m section coal pillar of 14321 working face of Gubei Coal Mine as the engineering background,by constructing the double mechanical criterion of deviatoric stress-deformation energy,combined with numerical simulation and field monitoring,the evolution law and instability mechanism of coal pillar force-energy in the process of repeated mining are systematically revealed.The results show that the yield of coal pillar is controlled by deviatoric stress,and its peak value increases with the increase of working face advancing distance,and the stress concentration area migrates from asymmetric distribution to the center of coal pillar.When the deviatoric stress exceeds the strength limit of coal,shear slip failure is induced.The deformation energy of the section coal pillar along the dip direction shows a single peak distribution,and the peak position migrates to the deep part of the coal pillar with the increase of the advancing distance the deformation energy along the strike direction shows a multi-peak fluctuation,which is significantly affected by the periodic pressure.The penetration rate of plastic zone under bilateral mining is as high as 85%.When the peak value of deviatoric stress coincides with the deformation energy core area,the coal body enters the plastic yield stage,which verifies the disaster mechanism of force-energy co-evolution.