Abstract: To meet the experiments need of split Hopkinson pressure bar (SHPB) experiment equipment with three-dimensional different prestressed impact, some quadrate devices were designed, for instance, input bar, output bar, absorb bar and a changeover portion which can change form circular section into square section. It was the premise to ensure the device of power launch system and bullets from any change on the one-dimensional SHPB experiment device of China University of Mining & Technology(Beijing). A formula of wave propagation law was deduced for the changeover portion based on the theory of stress wave and the method of numerical simulation. The result indicates that the peak of waveform will be reduced when passing changeover portion and the waveform will be stable when into the square bar. The longer of the changeover portion will be, the more of the peak of waveform will reduce. Comparing the calculated results, it is found that 500 mm is the reasonable minimum length of the changeover portion.