Identifying　the　weak　link　of　structural　stiffness　is　one　of　the　important　issues　in　improving　machine　tool　stiffness.　Aiming　at　the　cantilever　structure　in　machine　tool,　this　paper　proposes　a　new　weak　index　to　identify　the　weak　link　of　stiffness.　The　weak　index　is　based　on　the　modal　parameters　obtained　by　vibration　test　method.　In　this　method,　the　weak　parameter　matrix　is　obtained　by　multiplying　the　difference　between　the　reciprocal　of　the　flexibility　matrix　of　the　overall　structure　and　the　stiffness　matrix　of　the　overall　structure.　The　diagonal　elements　of　the　weak　parameter　matrix　are　extracted　to　obtain　the　weak　index.　And　the　change　of　weak　index　is　adopted　to　identify　the　weak　link　of　structural　stiffness.　The　method　of　element　stiffness　reduction　is　used　to　simulate　the　weak　link　of　stiffness,　so　as　to　verify　the　proposed　method.　Finite　element　simulation　is　used　to　verify　different　noises　and　single　and　multiple　stiffness　weak　links.　The　method　is　verified　by　numerical　examples　of　cantilever　beam　and　elastic　support　beam　structure.　It　has　higher　recognition　accuracy　under　noise　interference,　which　is　more　advantageous　than　the　flexibility　difference　method.　Finally,　an　experimental　study　on　the　elastic　supported　cantilever　beam　structure　and　the　tailstock　of　machine　tool　are　carried　out　to　verify　the　effectiveness　of　the　method.