Abstract: To study the safety of overlying strata by backfilling in the water preserved mining under thick alluvium and thin bedrock,the CT101 coal mining face of Wugou Colliery was chosen as the research site. By employing theoretical analysis,numerical simulation and drilling detection to analyze the stability of water-resisting key strata,revealing the destruction rule of overlying strata in working face mining with filling,and analyze the safety of mining. The results show that water-resisting key strata remains stable when the thickness of coal seam is 6 m and filling rate is 85% ;the downward cracks with bridging are distributed mainly over both the ends of working face. As the working face advan- cing,downward crack depth periodically increases and decreases. The depth of downward crack is 5. 5 m,and the height of upward crack is 6. 41-11. 85 m,the thickness of the aquifuge is 17. 38-23. 97 m,working face can be mined safely. The study provides a reference for the safety analysis on mining under similar mining conditions.