Abstract: Whether or not the dynamic parameters of earth rockfill determined in the laboratory or in situ are corresponding to the reality is a matter of concern. A back-analysis approach for the dynamic parameters of earth-rockfill based on response spectra and acceleration peak is presented. According to the acceleration response information of Liyutan Dam in the Chi-Chi earthquake, and by applying the back-calculation approach, the dynamic parameters are obtained. The results demonstrate that the proposed method is feasible, and the computational accuracy basically satisfies the practical engineering requirements. The dynamic shear modulus coefficient K of the dam materials obtained from the laboratory dynamic triaxial tests is small and should be revised.