Abstract：Through a steel beam damage experiment, the validity of the structural damage alarming method based wavelet packet energy spectrum is proved. This experiment has four cases, one of which is the initial case and others are three damage cases. Dynamic responses of a steel beam are measured and decomposed by wavelet packet and energy varieties of characteristic frequency bands are investigated. Experimental results reveal that three damage cases can be alarmed by the damage alarming indices ERV and ERVD and that ERVD varies slightly along with different measurements of the same damage case. The structural damage alarming indices based on wavelet packet energy spectrum are sensitive to damages, robust to noise and can be applied to actual structures.