Gob-side entry driving with small coal pillars is an effective means to control rock burst disasters. By establishing the motion mechanics model of the overlying rock structure on the free side of the gob-side roadway, the expressions of the rotation angle of the overlying key block and the support force of the coal pillar after the roadway excavation are obtained. Combined with the numerical simulation, the influence of the width of the coal pillar on the rotation angle of the overlying key block and the stress evolution law of the coal pillar is studied. The results show that with the increase of the width of the coal pillar, the rotation angle of the overlying key block decreases gradually, and the relationship between them is exponential function. The supporting force of the coal pillar increases exponentially, and the peak stress of the mining side does not change significantly, but the distance between the peak position and the coal wall of the mining side increases gradually. With the increase of the thickness of the coal seam, the rotation angle of the overlying key block increases linearly, and the supporting force of the coal pillar decreases exponentially. It is determined that the width of the coal pillar in the 6305 working face of Xinjulong Coal Mine is 4.5 m. The deformation of the surrounding rock of the roadway is small during the mining process. The microseismic events are mostly concentrated in the coal pillar area, and there is no large energy event in the overlying rock on the free side. The width of the coal pillar is reasonable and contributes to the slow release of roof energy.