The seismic response of MDOF frame was analyzed by HilbertHuang Transform. The results showed that the nonlinear time variant characteristics of MDOF frame under seismic load was hard to obtain by HHT, because of the complicated frequencies in the seismic response of MDOF frame. Because the vibration of MDOF frame is based on the low order mode shape, in this paper, considering the first order mode shape, the dynamic characteristic of MDOF system was analyzed by introducing the equivalent singledegreeoffreedom system. The results showed that the time variant dynamic characteristic of MDOF frame could be obtained by the instant excellence frequency of equivalent singledegreeoffreedom.