///Using Hilbert Huang Transform (HHT), the authors studied the timefrequency characteristic of observational data of strong motion recorded by the strong motion response arrays of the gravity dam in Shuikou hydropower station in Gutian ML4.6 earthquake and the structural dynamic characteristic of gravity dam. The conclusions were obtained as follows: 1) the frequency of strong motion record energies was centralized between 0 and 15 Hz while its time interval was centralized between 10 s and 15 s; and 2)the dominant frequency along the stream direction of concurrent gravity dam was 3.7 Hz. The results have certain practical significance for the understanding of the ground motion characteristics in near field of reservoirs earthquake and the seismic response of gravity dam.