Abstract: Floor principal stress difference distribution characteristics of 8# coal pillars was simulated by FLAC3D and theoretical analysis. Research results are as follows: ① The principal stress difference near coal pillar edge transfers to floor in 45 degrees; the farther distance to coal pillar is, the wider the diffusion range is, and it decays gradually. ② When the pillar is narrow, influence depth of midline and edge is shallow. When the coal pillar has increased with a central elastic core, floor varies obviously and influence depth is big, and if the coal pillar is wide enough, the middle part is as stress of primary rock. ③ Principal stress difference in midline and coal pillar edge decreases to depth in the form of negative exponent. ④ In the same horizontal plane, principal stress difference are distributed as saddle shape. ⑤ With pillar width increasing, principal stress difference in the coal pillar midline and edge increases at first and then decreases; however principal stress difference peak position in coal pillar edge outer margin decreases at first and then increases. It has proved that: ① Inboard-type layout could avoid principal stress difference peak, and roadway is in the low principal stress difference environment;② Enough inboard distance could avoid major principal stress difference change rate. With in boardtype layout of 20 m in 9207 belt roadway, the average principal stress difference is below 1.5 MPa. Both sides maximal relative convergence velocity is 6 mm/d; roof-to-floor maximal relative convergence velocity is 7 mm/d; Self-stabilization period is 10 days; both sides relative convergence is 87 mm and roof-to-floor relative convergence is 97 mm.

Key words: coal pillar, floor, principal stress difference, inboard-type