Abstract: The traditional predicting method of water flowing fracture height sometimes makes some discriminating errors because of its homogenization classification of roof lithology,its ignorance of the influence of different key stratum structure,and near-linear relationship between mining thickness and height of water flowing fracture based on the tra- ditional formula. In this study,the variation characteristics of water flowing fracture with changing mining thickness were investigated by way of theoretical analysis and engineering detection. Results showed that both mining thickness and key stratum structure could influence the height of water flowing fracture,which showed a stepped development characteristic but not continuous or even near-linear variation when mining thickness changed. The thicker the key stratum and the layer it controlled was,the larger the drop height was,and the more obvious the mutagenicity the height of water flowing fracture was. According to some key strata structures with certain characteristic,the height of water flowing fracture wouldn’ t change with changing mining thickness if the changed ranges of mining thickness didn’t exceed the drop height range of water flowing fracture. Namely,the decrease of mining thickness couldn’ t re- duce the height of water flowing fracture,and vice versa. The research results were verified by the engineering detec- tion result obtained from Tingnan coal mine. The height of water flowing fracture was 140. 2 m and 148. 3 m with the mining thickness 7. 5 m and 9. 0 m respectively in No. 206 working face,and the fracture have both reached the bot- tom of Yijun aquifer. These results convincingly proved that the increase of mining thickness in a certain range did not significantly change the height of water flowing fracture. On the premise that the height of water flowing fracture didn’ t increase obviously,the maximum mining thickness could be increased to 14 m theoretically in the mining condition of No. 206 working face. Determination method of reasonable mining thickness using fully mechanized top-coal caving mining method under aquifer should be pro-posed,and then guided the practice of preventing roof water disaster at Tingnan coal mine. According to the characteristic of key stratum structure in No. 207 working face,the scheme that the top coal could be entirely mined out was proposed. Finally,the actually total mining thickness of No. 207 working face was 9. 0 m and about 710 thousand ton of additional coal was mined,and the coal recovery rate was effectively im- proved.