Abstract: Last impacting velocity of the piston of pneumatic DHT hammer contacts nearly capability of pneumatic DHT hammer.FEM model of the piston is set up with FEM software ANSYS.Under different last impacting velocity,the stress and strain of the piston are analyzed.The conclusion is presented such as the stress of the piston increases with the increase of the last impacting velocity,but the change is nonlinear,and the last impacting velocity should be limited strictly.