Abstract: Shallow coal seam in west China mining induced deterioration of surface ecosystem. the root of surface diving drains is the instability of water resisting groups caused by mining.The domestic research on water stability of water resisting groups, research has focused on the "Water-flowing Fractured Zone", ignoring the affect of "downward cracks" to the stability of water resisting groups in subsidence basin edge tension band. To protect the shallow diving through a rational design of the back filling mining parameters,this paper researched on mechanical model of "boundary pillar?filling strip?water resisting groups" and established formula of "upward crack" and "downward crack" height, Laid certain theoretical foundation of solving water level drawdown of Sara Wusu group aquifer in north Shanbei jurassic coalfield.

Key words: DownwardCrack, WaterResistingGroups, ShallowSeam, Strip-Filling