A systematic discussion is made on the theoretical basis and innate characters of the structural damage alarming based on wavelet packet energy spectrum with the multiscale decomposition theory of the dynamic structure. The state equation and observation equation under various analysis scales are derived using wavelet packet analysis together with the wavelet packet decomposition of system matrix and observation noise

which reveal that structural damage alarming using wavelet packet energy will have preferable damage sensitivity and noise robusticity. The effectiveness of structural damage alarming for long-span bridges is subjected to structural damage and observation noise; structural damage alarming parameters are constructed according to the wavelet packet decomposition of noise energy.