Abstract: From the study of casing stress and strain in steam in jection wells,two methods of calculation are proposed,one for wells transformed for injection from production with or without cemented casings,and the other for calculation of the strength of pre-stressed casings in medium-deep wells.It is pointed out that when oil or gas wells are changed into steam injection wells,reliable measures for heat insulation must be taken and casing strength should be checked with heat intensity and heat conductivity.For wells specially designed for steam injection,pre-stressed casings should be used as fast as possible to reduce casing stress in steam injection.