In order to study the influence of drill diameter and working medium load stiffness to an electric hammer impact system, the force and drilling efficiency were calculated with numerical computation method of mechanical impact system. In the process of computation,seven kinds of drills and six kinds of working mediums were adopted.It shows that drill diameter and working medium load stiffness have no effect to the stress in the punch hammer or punch pole, but they have an important influence to the stress in the punch drill. With the increase of the diameter or the increase of the working medium load stiffness, drilling efficiency decline.For the same type of other electric hammer’s design and selection,the results have a theoretical guidance significance.