Characteristics impedance is a very important physics parameter for many applications in rock fragmenting and percussive drilling. When the shape of the elastic bar is very complicate, it is difficult to calculate the characteristics impedance of the elastic bar with its definition. A method which uses the incident wave and reflected wave to calculate the characteristics impedance of elastic rods is presented. Firstly, the relationship of the incident wave, reflected wave and the characteristics impedance of elastic rods is studied on the basis of the reflection and transmission laws of elastic waves and, the resolution model is established; Then impact experiments were carried out in which a drilling bit with step cross-section was designed. The signal processing of eliminating polynomial trend item and discrete wavelet de-noising were done to stress signals before calculation to decrease the error. It is proved that the method is adequately accurate from the testing results.