External transient impact loads widely exist in the service environment of various engineering structures, and urgent engineering and military application requirements call for research on the theory and method of impact identification. This manuscript proposes a method to identify the impact event. By employing the dispersive property of wave propagation through the waveguides, a dispersion-based average power spectral density (PSD) curve for quantitative representation of the dispersion is established, and its evolution law is investigated to apply the identification of impact events. The critical parameters of the average PSD curve are obtained by analyzing the response of the structure from the perspective of mechanics. The effectiveness of this method is verified through the development of the triple-successive-impact split Hopkinson bar and the triple-successive-impact experiment. Through the method proposed in this manuscript, a kind of impact identification under multiple impact loads, which is not convenient to measure directly by the sensor, is realized, especially in the case of highly transient external impact.