Abstract: Abstract：In order to study the state of coal pillars left over from the upper protection layer mining in deep wells when the lower protection layer is mined, the coal pillar’s ultimate strength calculation, numerical simulation and on-site detection are used to conduct simultaneous research plans to communicate with Pingdingshan No. 4 Mine 15-23160 The section coal pillars with a width of 4 m on both sides of the roadway have been studied, and the conclusions are as follows: 1) The theoretically calculated ultimate strength of the section coal pillar is 26.0 MPa; 2) Coal pillar 1 and coal pillar 2 are in the Ji 15 coal seam and 16.17 The maximum vertical stress that the coal seam withstands during mining is 131.1 MPa and 85.1MPa, which far exceeds the ultimate strength of the coal pillar; 3) The maximum residual height of the remaining coal pillars detected by the two detection groups on site are both 0.64 m, and the height loss reaches the original coal 60% of the height of the pillar, the collapsed coal body rushes to the goaf on both sides of the coal pillar and the 15-23160 connecting road; 4) The residual coal pillar will not form an obvious stress concentration area on the protected layer in the lower part, and the protection effect is in there is continuity in the tendency.