Abstract: In order to judge the stability of irregular coal pillar accurately, this paper proposes the idea that the shape index and area of coal pillar are the factors influencing the stability of coal pillar. By means of mathematical statistics and case analysis, the paper suggests that the coal pillar shape is more complex, the safety coefficient is smaller, and the coal pillar area is bigger, the safety coefficient is bigger. There is a quantitative relationship between the safety factor of coal pillar and the shape index and area of coal pillar: in the mining depth of about 150 m, coal thickness of about 2.2 m, mining width of 20 m to 30 m, coal pillar area under 3 000 m² condition, the safety coefficient of the irregular coal pillar K and its shape index S and area A function relation expression is K＝8.799 9S^-0.9+0.056 1ln（A）－1.773 4. The results show that the shape index and area of coal pillar are important factors that affect the stability of coal pillar, they can be used as an important index to calculate the safety factor of irregular coal pillar.