Abstract: The specific yield of aquifer at the bottom of thick unconsolidated layers, thickness of clay layer at the bottom of thick unconsolidated layers, thickness of strata, water flowing fracture height and mining height are five key factors for safety mining under thick unconsolidated aquifer with thin bedrock. By analyzing and processing the key factors, four junction points influenced the mining safety were found out, namely, the unit water inflow in bottom aquifer, the ratio of overburden thickness and mining height’s square root, the ratio of the thickness of bottom clay layer and mining height, and the ratio of water flowing fracture height and mining height’s square root. Evaluation factors were divided into five grades, to quantify the mining safety. The information entropy and unascertained measure theory were introduced to establish comprehensive evaluation model. 105 drilling data in one mine were used to evaluate mining safety, divide and predict the safety mining area according to the evaluation results. The evaluation and prediction results are quite consistent with the engineering practice and existing research results. The results show that the unascertained measurement model is feasible and effective under the studied coal seam condition.

Key words: thick loosened aquifer with thin bedrock, safety mining area, unascertained measure, entropy