The　presence　of　service　torque,　which　is　generated　after　tightening,　directly　affects　the　mechanical　properties　of　bolted　joint.　In　this　work,　the　mechanical　behavior　of　bolted　joints　under　torsional　loads　is　investigated　using　finite　element　analysis　(FEA).　Models　with　two　plates　and　multiple　plates　are　established　separately　to　investigate　the　mechanical　properties　of　different　kinds　of　bolted　joints.　Three　analysis　steps　involving　the　entire　process　from　tightening　to　torsional　loading　are　adopted,　namely　tightening　step,　initial　loss　step　and　loading　step.　Torsional　loads　in　two　directions　along　the　thread　axis　are　considered　and　defined　as　loosening　torsional　load　and　tightening　torsional　load.　The　clamping　force,　as　well　as　the　friction　torques　of　various　contact　surfaces,　of　the　bolted　joint　are　extracted　in　all　the　analysis　steps.　Different　scenarios　of　friction　torque　combinations　for　bolted　joints　are　obtained　by　means　of　adjusting　the　friction　coefficient　of　the　corresponding　contact　surfaces.　On　this　basis,　the　mechanical　properties　of　bolted　joints　and　the　corresponding　torque　thresholds　are　extracted　on　various　scenarios.　FEA　results　showed　that　how　the　service　torque　makes　the　directional　characteristic　of　the　mechanical　properties　of　bolted　joints　under　torsional　loads.　Moreover,　the　service　reliability　of　bolted　joint　can　be　further　improved　based　on　the　directional　characteristics　and　the　extracted　torque　thresholds.