Because it is very important to study the coal pillar stability, a lot of methods to do this have been proposed. But most of those methods do not consider the nature of coal material and only study the coal pillar that is level. To solve this problem suitably, here a new method is proposed. In this method, the plastic softening nature of coal material is considered. And also, the coal seam pitch is considered in our analysis. Based on real deformation of coal pillar and the previous study, the limiting equilibrium method is applied. At last, the rationality of our method is verified through a strip mining engineering example. And the results show that, the computing results of our method are coincided with measuring results very well and our method can be applied in real engineering practice very well.