Abstract: A mechanical analysis on liquid medium filled in double-layer combined casing was conducted. An expressing equation of pressure transfer coefficient of the medium was deduced. The alteratlon law of the equivalent stresses on the inside wall of the combined casing with the elastic module of the volume of the liquid K and with the external pressure Pw when K=7.7×10-3 MPa were also analyzed. Therefore a theoretical basis was created for study on annular medium of the combined casing and the damage prevention of the casing.