Abstract: A large amount of scatter in bolt preloads is observed when bolting up a high temperature and pressure vessel head flange connection.In this study,a method to test the process and basic operation is pro-posed,by measuring the strain of bolt by stick to strain gage on the surface of a bolt;and a mathematical model of sine and exponential is designed to predict the scatter in bolt preloads,and thus to explore the appropriate of way on tightening.Experimental verification is provided for the validation of the mathematical model.Experimental and analytical results are presented and discussed.It is found that the stress of bolt meets the sine rule along the flange weeks;the single bolt meets the exponential decay law in the process of tightening.