In this paper we calculated the reflection spectrums corresponding to different incident angles on a thin layer mould, and compared the spectrums which response to different velocity ratios, Poison s ratios and layer thicknesses. We also showed that the AVO characters in frequency domain are similar to those in time domain, and differential tuning is the main factor affecting spectrum-versus-offset. We proposed that we should use spectrum extremum and extremun frequency as characteristic parameters in spectrum-versus一offset