Ground surface in Yonglong coal mine is hilly terrain. There is a Yinshigou reservoir on minefield. Deep coal seam mining under the reservoir has an influence on the deformation of oversize normal fault, and can cause severe mine water-inrush. Using numerical simulation software UDEC, the paper studied the characteristics of plastic zone and stress field in front of coal face in the hanging wall of the fault, while the coal face advance distances from setting-up room to coal face line were different. The results showed that while the distance from coal face to the fault i. e. the width of fault-protected pillar, was 80 m, the front abutment pressure had less influence on two sides rock mass of the fault, the fault reduced the vertical stress of its surrounding rocks; The surrounding rocks in the vicinity of fault were in a same vertical displacement contour, whose displacement was very smaller. Reasonable fault-protected pillar width of the numerical study was much closer to the average water-proof coal pillar width based on coal mines water prevention regulations of China. Consequently, while the width of fault-protected pillar is more than 80 m, coal seam mining in hanging wall of fault has no influence on the fault, and coal seam safely mining under the reservoir could be carried out.