Abstract: s:Addressing the downhole operation by means of connecting the casings via butt-welding process,the X80 welded downhole casing is developed.Three types of steels for making the said casing are designed.The steels are trail produced with the same melting and rolling processes,and properties of the steel pipe are adjusted up to the standard range for the X80 steel grade via proper heat treatment process.The metallographic structures of the three steels are analyzed,and their mechanical properties and welding ability are evaluated as well,based on which,the best of the three steel types is determined,and put into industrialized production.The application status of the product is investigated.Relevant analysis leads to the conclusion that the welded X80 downhole casing as developed possesses high level pipe body and weld joint performance,having been met butt-welding process requirements for the welded downhole casing.The casing has been successfully used for a certain domestic gas storage.