Preload　relaxation,　caused　by　elastic　interaction　between　tightened　bolts,　is　one　of　the　major　reasons　for　non-uniformity　of　pretension　force.　But　how　to　accurately　assess　the　residual　tension　of　a　bolt　is　still　extremely　difficult.　This　paper　considers　two　types　of　multiple　bolted　joint　and　six　kinds　of　tightening　sequence.　Based　on　the　deformation　compatibility　equation　and　elastic　interaction　stiffness,　the　generalized　mathematical　model　for　evaluating　residual　preload　of　bolts　is　established.　Certain　model　parameters　are　obtained　through　numerical　simulation.　To　analyze　the　influence　of　initial　tightening　torque,　spacing　between　bolt　holes,　bolt　material　and　gasket　material　on　the　elastic　interaction　stiffness,　experimental　devices　and　test　procedures　are　developed.　Furtherly,　preload　relaxation　models　are　validated　by　tests　conducted　on　six　types　of　multi-bolted　joint　with　the　same　surface　roughness.　Results　of　the　proposed　mathematical　model　of　pretension　force　show　a　good　consistency　with　those　of　corresponding　experiment.　(C)　2019　Elsevier　Ltd.　All　rights　reserved.