Abstract: Tubing and casing connections are widely used in oil and natural gas industry.Their seal performance is very important for circumstance and production safety in oil and gas well.In this paper,the threaded connections in the tightening condition are taken as research objects and a calculation approach to the squeezing force on the shouldered seal face of a conical threaded connection is established in the elastic range.By the method of micro contact analysis on sealing rough surface,API round threaded casing connection is taken as the calculating sample,and the influences of tightening torque (tightening cycles),roughness and initial seal clearance on the seal performance of the shouldered face are numerically simulated.And the concrete variation curves are obtained and analyzed in detail.