Abstract: In actual application,Poissons ratio is mainly obtained from indirect inversion,and its cumulative error is obvious.Aiming at the problem,based on elastic impedance inversion theory,firstly,the general formula of conventional elastic impedance equation and general extraction method of lithologic parameters;then,a new AVO approximation was derived;thirdly,a new elastic impedance equation involving Possions ratio parameters was proposed.Model testing and actual application results show that elastic impedance inversion by using the new AVO approximation can directly obtain Possions ratio,which reduced cumulative error,and improved the stability and anti-noising ability,moreover,the accuracy of the Possions ratio is higher.