Abstract: In terms of the issue of the abnormal gas emission during periodic weighting in the fully-mechanized sublev- el caving mining of extremely thick coal seam,based on the “O-X” type breaking of hard roof,the impact process of hard roof breaking to the goaf is analyzed from the view of energy to get the relationship between the axial force of bro- ken block and the broken expand coefficient of caving coal-rock in goaf. Besides,combined with the effect of the top- coal recovery rate and the broken expand coefficient of caving coal-rock on the volume change of goaf during hard roof breaking,the formula for gas emission increase in goaf during periodic weighting is established,and its results are com- pared with the measured data. The result shows that the volume of goaf is reduced by the breaking of hard roof in the fully-mechanized sublevel caving mining of extremely thick coal seam,so the high concentration of gas in goaf will be poured into the working face which leads to abnormal gas emission during periodic weighting. The theoretical calcula- tion of gas emission increase in goaf for the 8214 working face of Tashan Mine is 14. 5 m3 / min,and the error is be- tween 1. 79% -8. 83% compared with the measured data. The results are consistent,which verifies the correctness of the formula.