Abstract: In order to improve the coal recovery rate and ensure the safety of the tunnel, the coal pillar size is generally designed in coal mining. The coal pillars are divided into two types: "protecting roadway" and "protecting surface". We analyzed the elastic-plastic of the two types of roadway respectively and obtained the ultimate strength calculation formula considering the effect of intermediate principal stress based on the unified strength theory. We obtained the calculation formula of yielding width and coal pillar size. By using this formula, the pillar size of "protecting roadway" at 8214 working face is calculated, and the calculated results are compared with the numerical calculation. The width of the coal pillar with 10 m is appropriate.