Abstract: In this paper,the size effect of irregular pillar has been studied through the effect of width-to-height ratio based on the pillar intensity theory and the numerical simulation.The study shows that in the situation of 5 m to 10 m wide coal pillar,there is not large pressure over it and if it can be kept in integer by certain supporting measures,it still can be maintained in steady state rely on its residual strength;when t the width of the pillar is around 15 m,it is at yield and there is not elastic state region in the center and at the same time,the peak stress is over the coal pillar and it can not transfer to coal seam.So the medium size coal pillar goes against the stability;when the width of the coal pillar surpasses 20 m,though the pillar has still suffered the peak of the vertical stress there are plastic zones in both edges of the pillar.In this situation,the pillar has sufficient bearing capacity to maintain itself in a steady state,but it goes against the saving of coal resource;the coal pillar intensity basically maintains constant as the coal pillar width surpasses 23 m,which provides a certain reference value to the coal pillar design.