The　loosening　is　one　of　the　main　failure　modes　of　bolted　joints.　Currently,　researchers　only　focused　on　bolt　loosening　behavior　under　simple　axial　or　transverse　loads.　Understanding　the　loosening　failure　mechanism　of　bolted　joints　under　complex　load　is　important　for　improving　reliability.　A　multi-directional　loading　test　device　was　designed,　and　bolt　loosening　experiments　were　conducted.　The　effect　of　load　direction　and　amplitude　on　bolt　loosening　behavior　were　investigated.　The　results　indicate　that　bolt　loosening　can　be　divided　into　two　stages.　Load　direction　and　load　amplitude　are　critical　factors　determining　the　loosening　trend　in　the　later　stage.　In　addition,　a　refined　finite　element　model　of　bolted　joints　was　established　to　verify　and　explain　the　experimental　results.　The　FEA　results　show　that　the　relative　sliding　between　internal　and　external　threads　and　the　stress　redistribution　on　the　threads　are　the　main　reasons　for　the　bolt　loosening.　The　smaller　the　angle　between　the　load　direction　and　the　tangential　direction　of　the　contact　surface,　the　relative　sliding　more　likely　to　occur.