The mechanical analysis of the various components of the flared coupling was carried out to determine the contact state of the contact surface during the tightening process.The results show that during the tightening process of flared coupling,there are three relatively sliding surfaces among six contact surfaces,which in respect are the screw contact surface between nut and flared coupling,the ring contact surface between sleeve and nut,and the cylindrical contact surface between sleeve and nut.Based on relationship between the preload and tightening torque of common bolt connection,the theoretical formula of the relationship between the preload and tightening torque of the flared coupling was obtained.The simulation model of flared coupling (includes helix angle and thread) was set up by Hexahedral mesh modeling method,and the relationship between the preload and tightening torque of two kinds of assembly forms was studied as clearance and transition fit.The results show when the cylindrical contact surface of the sleeve and the nut is in clearance fit,the theoretical formula of the relationship between the preload and tightening torque is very accurate.When the contact surface is in transition fit,the theoretical formula may cause significant errors,and a correction factor is required to be introduced. 〖WT8.75HZ〗Keywords:〖WT〗flared coupling;tightening process;relationship between the preload and tightening torque;hexahedral mesh