Abstract: Appropriate protective coal pillar was left to protect the lateral roof roadway in No.1 Mine of Pingdingshan Coal Mine. However, severe deformation of the roadway occurred after the working face was mined. Aimed at this problem, the numerical model was applied by using UDEC^2D combined with geological conditions on spot. The result showed that the angle between the horizontal line and the line connecting the 10 mm subsidence boundary point and the border point of gob was defined as internal strata displacement angle. With the declining of the buried depth of the roof strata, the corresponding internal strata displacement angle became larger. The internal strata displacement angle corresponded to the Dingwusan auxiliary roadway was 40°, so the necessary width of the protective coal pillar was 102 m, which was more than the actual width. In result, severe deformation of the roadway occurred because of the effect of working face's mining. The numerical simulation result was consistent with the measurement on spot, and this indicated that it was more accurate to design the width of the protective coal pillar for roadway by using the internal strata displacement angle.