Abstract: The water accumulation in goaf and coal rock interaction will weaken the strength of the coal pillar in the section and cause gradual destruction and failure of the coal pillar. The interaction of water and rock is the key factor that must be considered in the design of the reasonable width of the coal pillar. The uniaxial compression experiment and theoretical analysis are carried out based on the engineering background of the coal pillar design between mining area 31 and 33 of a mine in Xinjie mining area, Ordos, Inner Mongolia. The results show that water-rock interaction has a significant impact on the weakening of coal strength parameters. The width of the plastic zone at the side of the water accumulation in the section coal pillar expands with the increase of the weakening degree of the coal body strength. Based on the basic conditions for the stability of the section coal pillar, the reasonable theoretical width of the section coal pillar is 53.62 m. Using FLAC3D to simulate the process of water-rock interaction, the paper analyzes the stability characteristics of coal pillar with different widths. The results show that when the width of the coal pillar is small, the weakening effect of water accumulation in goaf has stronger destructive capability to the elastic core area with higher stress concentration. With the increase of coal pillar width, the stress concentration degree in the elastic core area decreases. The area where the vertical stress at the water accumulation side of the goaf is lower than that of the original rock increases. The stress concentration distribution on both sides of the coal pillar tends to be uniform, and the weakening effect of water accumulation in the goaf on the elastic core area is no longer significant. Based on the results of theoretical calculation and numerical simulation, the width of coal pillar is determined to be 70 m. The engineering application results show that the coal pillar with width of 70 m can effectively bear the roof pressure. The deformation of the roadway surrounding rock is small, the stress of the anchor cable is stable, and the safety production of the mine is guaranteed.