Abstract: In order to tackle the problem of reasonable width of narrow coal pillar of 20103 haulage entry in Wangjialing Mine, a research method taking the third invariant of deviatoric stress J₃ as the evaluating criteria was proposed. The third invariant of deviatoric stress is used to characterize the strain behavior of the medium point and is determined by means of numerical simulation to choose the optimal width of coal pillar. The conclusion was drawn through the scheme of different pillar width comparison as follows: when the width of the 8 m column is wide, the surrounding rock of the roof above solid coal is in a state of low compressive strain, and the surrounding rock above the coal pillar is a plane strain or a low tensile strain state. Finally, the reasonable width of narrow coal pillar was determined to be 8 m in the 20103 haulage entry. The numerical simulation and field application show that the coal pillar with 8 m still has a elastic core of 2 m to 3 m under the strong influence of severe fully mechanized top coal caving mining. The instability of narrow coal pillar is not found in the field and the service condition of the haulage entry is good.