API Standard threaded casing connections can’t meet the application demands of sealing property under the high temperature and high pressure on account of the structure design defect. The seal structure of premium threaded connections of the matching of arc surface and conical surface was proposed, which adopted barb anti-torque shoulder of auxiliary seal effect.The seal property of structure can be improved by strictly control of the contact pressure of sealing face, and optimal selection of taper, strength, interference of sealing faceThe result of full-scale test shows that the method of designing premium threaded seal is feasible and the designed threaded casing connections meet the application requirements of sealing property.