Abstract: In order to explore lateral growth, extension and termination laws of coal seam floor mining under pressure, Chengzhuang Coal Mine of Jincheng Anthracite Mining Group is selected as engineering field. Through theoretical analysis and calculation about the damage zones in different kinds of coal seam floor and field tests, a comprehensive research about transverse evolution laws of coal floor mining-induced fractures is studied. The results show that the horizontal damage length (30 m) of coal seam floor based on plastic theory and the result of water injection tests (from 28 m to 31 m) fits each other very well, which verifies the lagging effect of fractures in coal seam floor under pressure, and explains lagging outburst phenomenon of floor water in some old gobs or concealed structure areas, and effectively guides the safety production.