Abstract：Stable structure of main roof couldn¢t be formed because of sliding instability and crushed support during fully-mechanized mining with top coal caving in thin bedrock and thick surface soil. By defining thin bedrock through “three belts” which are decided by strata control，it is found that “three belts” are closely related to mining height and rock mechanical properties. The overlying strata movement rules are studied by laboratory experiment，theoretical analysis，numerical simulation and in-situ measurement. The result indicates that bedrock thickness and mechanical properties and thickness of surface soil are the key influential elements for the stable structure of “stacked layer of blocks”，while the other conditions remain the same. The thick clay in surface soil that owns large permissible bearing capacity can form steady structure with the composition of thin bedrock，so it reduces minimal thickness of bedrock to ensure structure stability. According to the practical condition of the first face in Sima coal mine，the mechanical model of structure which is combined by thick clay and thin bedrock is built. It shows that the immediate roof and top coal can be regarded as “similar zero rigid body” by using plastic and elastic combination body. Through designing the mechanical features of surface soil and change of thickness in bedrock，the rules of the vertical stress，bearing pressure and top coal convergence are simulated and studied，and the minimal bedrock thickness is ensured. The minimal bedrock thickness is about 40 m in loose surface soil condition；when the clay thickness is 40 m，the minimal bedrock thickness is 20 m；and when the clay thickness is 30 m，the minimal bedrock thickness is 30 m. Through the study，it can be deduced that safety mining is feasible at the first face，and it has been proven successfully in mining experiment.
