Abstract: In order to study the depth and position of downward cracks development with large area mining in coal seam, it took the bedrock clay shaped strata geological conditions of shallow coal seam special water conversation mining area as the research background, with numerical simulation experiment, it studied the migration laws of downward cracks development characteristics and impermeable rock group, and established the mechanical model of three adjacent working faces "boundary pillar - goaf - impermeable rock group", then analyzed the formation conditions of downward cracks, thus deduced the theoretical formula of development height and development position of downward cracks.