Abstract: In order to analyze rock failure laws of floor in mining under pressure, choosing Chengzhuang Coal Mine as engineering field, through numerical simulation of floor damage of main coal seams in different depths and water-injection test detection of floor fracture development actual depths, a comprehensive research on floor fracture development laws in mining under pressure is made. The results show that the main influencing factors of coal seam floor damage in mining under pressure are the width of working face and the lithologies of coal seam floor. However, the depth of coal seam and the thickness of mining seam do not much effect on it. Moreover, numerical simulation is proved to be a reliable method to study the depth of coal floor cracks through certification of engineering test.