Presently, there are variously methods for calculating elastic impedance by different seismic elastic impedance inversion softwares, and seismic elas tic impedance inversion were often simply applied though some requirements hadn’t been discussed in detail, which causes yield difficulties for actual productions. Therefore, based on four typical types of gas-bearing sandstone models, calculation accuracy of elastic impedance（EI）, reflected impedance（RI） and generalized elastic impedance （GEI） are analyzed and compared. Moreover, based on Biot-Gassman theory, fluid replacement is applied for gas-bearing sandstones model advanced by Ostrander, and the sensitivity to fluid of different elastic impedance is discussed when water saturation changed. The following conclusions can be obtained through the calculation of four types of gas-bearing sandstone models: As for Class Ⅰ and Class Ⅳ, Zoeppritz equations can be substituted forthe EI, RI and GEI when the incident angle is less than 40 ;As forClass Ⅱ, Zoppritz equations can also be substituted when the incident angle is less than 30 ; However, these three kinds of elastic impedance can not be applied for Class Ⅲ; The idea of the partial derivative is applied for the gas-bearing sandstone models advanced by Ostrander, the result shows that, RI is more sensitive to fluid than EI and GEI as incident angle is bigger.