Abstract: Based on the distribution and size of the pillars which resembled as Chinese character “品” and the size of the arch in the plastic zone boundary above the goaf,the average overlying load of pillar under steady state is calculat- ed. Combined with the average overlying load change law of the adjacent pillar after the fully plastic instability of pil- lar,the unit instability probability of pillar is defined. The renormalization group method is used to divide pillar sys- tem,and one dimensional and two dimensional cellular models are established. The probability density function (PDF) of the pillar instability is expressed as the distribution of Weibull function and its fixed point equation is solved,then the reasonable solution are selected as the critical probabilities of pillar instability. Based on the geologic data of Shige- tai Mine,the FLAC3D numerical simulation model is established to analyze the cellular model and the instability prob- ability distribution of pillar system,and the stability of pillar system in the room and pillar mining goaf of Shigetai Mine is evaluated.