Abstract: In order to reveal the pillar rib outward sliding mechanism of entry in No. 5- 2 coal seam with 20 cm mudstone interlayer in Sandaogou mining area, thro ugh borehole observation, convergence observation and mineral composition of X- ray diffraction analysis, it was determined that montmorillonite content was 55% and kaolinite content was 24% in the mudstone. It was found that there were vertical fractures in the roof, and pillar rib damage area was larger. The entry convergence def ormation was mainly aroused by the adjacent face mining, and the outward sliding deformation was mainly occurred in the pillar rib. Based on the numerical analysis, th e mechanism of pillar rib outward sliding was that the lateral abutment pressure aroused by the adjacent face mining and led to pillar roof vertical fracture and the horiz ontal movement to adjacent goaf, and pillar rib along mudstone interlayer sliding and outward sliding deformation. Based on the discovery, the support method of long a nchor strengthen rib alone mudstone interlayer was put forward, and achieved successful application.