Abstract：Based on the analysis of petrofabric property of engineering in thick loose bed，water-bearing strata and water-resisting layer in ultrathin overlying strata，as well as weathered zone in basement rock，the maximum height of caving zone of overlying strata and the effective height of water flowing-fracture zone in the condition of different caving ratios are studied by using the model test of simulated material and numerical simulation. The results show that the content of clay minerals in weathered zone of basement is relatively high，which results in weak permeability and strong regenerated water tightness，which is of double functions of water tight and restrain of continuous development of the height of water flowing-fracture zone under mining conditions. The overlying strata bend wholly and sink slackly，and the effective height of water flowing-fractured zone is in accordance with the caving zone height of overlying strata. The study results are applied to the caving area 810# of Luling mine. The top-coal caving mining is done in waterproof coal pillars，in which experimental exploitation is successful in two working faces. The sum of 4×105 t coal is produced safely，and the recovery ratio is up to 86.7%，so that significant economical benefits are obtained.
