This　paper　describes　a　composite　method　of　calculating　multi-tooth　root　stress　and　evaluating　fatigue　life　of　flexspline　(FS)　in　harmonic　drive　(HD).　In　order　to　accurately　describe　the　stress　state　of　the　flexible　gear　root,　a　mathematical　method　combined　with　simulation　method　is　applied.　FS　is　divided　into　two　parts,　the　cup　part　and　the　teeth　part,　according　to　the　type　of　stress.　On　the　cup　part,　the　stress　is　caused　by　the　support　of　the　wave　generator　(WG).　The　bending　stress　is　analyzed　using　differential　equations　and　appropriately　set　boundary　conditions.　The　theoretical　analysis　of　meshing　teeth　in　a　harmonic　drive　is　brought　into　the　finite　element　model　of　FS　that　is　deformed　by　the　outer　contour　of　the　wave　generator.　The　simulation　is　conducted　under　the　action　of　torque　and　the　meshing　generated　stress　is　obtained.　Finally,　the　stress　spectrum　of　the　tooth　root　is　acquired　according　to　the　flexural　and　torsion　stress,　while　the　maximum　radial,　the　circumferential　and　shear　stress　are　obtained.　After　applying　the　fourth　strength　theory　to　calculate　the　Von-Mises　stress,　the　safety　factors　of　several　materials　are　compared　according　to　the　S-N　curve　and　fatigue　limit　formula.