Abstract: The instability of the bottom coal pillar induces the collapse of the coal pillar at the upward re-mining working face threatens mine safety, and ensuring the mining stability of the bottom coal pillar is the key to the safe recovery of left-over coal seam. In order to solve the problem of upward re-mining in No.6107 working face of Yuanbaowan coal mine, the law of stress distribution in the bottom coal pillars under the lateral clamping effect of the filling body is explored and the law of deflection of the shear stress trajectory under the lateral limit effect and its strengthening mechanism on the bearing capacity of the bottom coal pillars is revealed through theoretical analysis, FLAC^3D numerical simulation, and engineering practice. The correlation characteristics of the “vertical stress drop and dissipation energy concentration zone connection” of the mined coal pillar are found, and the dual indicator type of “vertical stress drop + dissipation energy concentration zone connection” is proposed to determine the instability of the mined coal pillars and the enhancement effect of the filling body strength on the bearing capacity of the bottom coal pillars is revealed. The results show that: The shear stress trajectory in the bottom coal pillar is deflected under the action of filling body clamping, and when the filling body clamping force increases from 1.0 MPa to 5.0 MPa, the same shear stress trajectory transitions from monoclinic penetration type to V-shaped, and the shear stress components on both sides of V-shaped form slip-inducing and slip-resisting sections and both cancel each other and the critical monoclinic shear damage stress increases with the increase of filling body strength. The critical monoclinic shear failure stress increases with the increase of filling strength. No.6 and No.7 coal pillars are destabilized, and the maximum dissipation energy in the two pillars develops from the upper left and lower right corners toward the center of the pillars, and finally forms a monoclinic through shear damage zone at the center. The overburden load is transferred to the No.4 and No.5 coal pillars when the No.6 and No.7 coal pillars are destabilized, and the No.5 coal pillar is closer to the destabilization zone so the dissipation energy core zone is connected earlier than the No.4 coal pillar. No.4 and No.5 coal pillars are also damaged as a whole, but their bearing capacity is slightly larger than No.6 and No.7 coal pillars, No.1, No.2, and No.3 coal pillars are not damaged and continue to support the overlying strata after the working face is pushed through. There is a significant correlation between the “vertical stress drop” and the “dissipative energy core zone connection” in the coal pillars at the bottom of the working face, and the process of vertical stress reduction corresponds to the process of gradually connecting the outer edge of the dissipative energy core zone in the upper left and lower right to the two cores. The “vertical stress reduction + dissipative energy core zone connection” can be used as a criterion for the destabilization of the coal pillar at the bottom. Increasing the backfill strength is beneficial for maintaining the stability of the floor coal pillars. Under the conditions of no backfilling and backfill strengths of 1 MPa and 2 MPa, advanced instability still occurs in coal pillars No.6 and No.7. However, when the backfill strength increases to 3 MPa, advanced instability no longer occurs in these two coal pillars. As the backfill strength increases, the vertical stress in the coal pillars first increases and then decreases, indicating that the load among the coal pillar group is autonomously redistributed, achieving coordinated load-bearing behavior among the pillars. Based on this, the dual index of “vertical stress drop + dissipation energy concentration zone connectivity” is proposed as the basis for discriminating the instability of mining coal pillars, and it is pointed out that it is not appropriate to take only the plastic deformation characteristics such as the elastic core zone proportion greater than 31% and the plastic zone disconnection as the discriminating method for the instability of mining coal pillars but should consider the synergistic evolution of vertical stress and energy dissipation in the whole bearing process of mining coal pillars. The study is expected to contribute to the development of the coal pillar instability and provide a reference for the evaluation of the stability of the mined coal pillar and the determination of the reasonable strength of its filling body.