Abstract:AVO method means that CDP gather is used to analyse relation between reflection amplitude and offset.In the light of the relation, we can calculate elastic parameter from seismic section so as to identify lithology. The analysis of theoreticai curve leads to the eonclusion that the location of polarity reversal depends on △σ The location of polarity reversal moves toward the low offset with the increase of |△σ| when ρ1v1<ρ2v2 .However, when ρ1v1>ρ2v2, the location of polarity reversal moves toward small offset with the increase of △σ, or toward big offset with the decrease of △σ, According to this regularity, we can make lithology inversion by making use of iterative method.