Abstract: The impact of the fault on the stress peak and the variation of plastic zone during coal bed mining was simulated by using FLAC5.02D numerical simulation software.Different fault dip，fault stiffness and the variations of the stress peak and the plastic zone under floor water pressure were analyzed.Meanwhile，according to the field measurement，the damage depth with or without fault is compared.The smaller the fault dip is，the higher the floor stress peak value and the larger the floor failure zone is.The larger the fault dip is，the smaller the damage zone is，eventually developing as “X”type.With the increase of the fault stiffness，it has no obvious influence on the floor plastic zone，but only improves water inrush channel from one side of the fault.With the increase of the floor water pressure，the plastic zone did not change significantly，and the volatility of the floor peak stress is not large.No fault exists，the floor failure depth is 0~17 m.The fault zone makes the floor failure depth increasing by 135%.

Key words: Plastic zone, Stress peak, Fault dip, Fault stiffness, Water pressure, Damage depth