Abstract: The movement characteristics of thick alluvial rock is the mutual coupling effects between thick loose layer and thin bedrock, specific performance is that if the height is sufficiently thick, loose layer will show cyclical characteristics of arch structure with the vary periodically of thin bedrock, and structural characteristics of the arch can avoid the presence of all the role of the overburden on the support. However, if the height of the loose layer thickness is not enough, to a certain extent in the mining, loose layer will show the integrity of collapse, and then the weight of the whole layer of alluvium will be delivered on bracket with a certain load factor, which may easily lead to increased pressure, or even bracket accidents. Based on this, an effective arch of a minimum thickness of loose layer was advised by 70 m, and the working resistance of support under the condition was calculated.