Abstract: To meet the need for designing the unconventional well structure of the sidetracking horizontal well as located in a certain oilfield in western China，the Φ139.7 mm×7.72 mm 140 ksi CST-ZT super-high strength flushjoint casing with premium connection is developed.Usingreasonable chemical composition design and effective manufacturing process control，satisfactory strength-toughness match of the casing is ensured，while the technical difficulty concerning the connection strength is successfully solved by means of the inner and outer upset structure. Relevant test results demonstrate that the casing features connection strength as equals to over 70%of that of the pipe body and excellent operation performances，and also has its all other properties up to or even higher than the designed indexes including thread-gulling resistance，tensile property，internal pressure resistance and collapse resistance，etc.Furthermore，the casing has passed both over-torque drifting test and bending drifting tests under operation conditions as simulated to the workshop ones.