Abstract: The transient response of a structure is usually ignored in traditional dynamics. Based on the theory of harmonic earthquake response and wavelet packets analysis, the transient vibration of MDOF systems subjected to real earthquake ground motion is derived. Response spectra and transient vibration rules of equivalent SDOF system are got. It is concluded that the maximum displacement of a long period structure could occur in transient vibration and the value could be greater than sympathetic vibration. By comparing the transient response of a long period building with that of a short period building, it is shown that short period structure is mainly controlled by steady state vibration and the effects of transient vibration on a long period structure is prominent. Therefore, transient displacement as well as sympathetic displacement must be considered in the earthquake-resistant design of long period structures.