Identification　of　the　vulnerabilities　in　the　structural　stiffness　is　one　of　the　most　crucial　issues　in　improving　this　property　of　machine　tools.　In　this　paper,　the　Flexibility　Matrix　Diagonal　element　method,　based　on　hammer　testing,　is　proposed　as　an　effective　approach　to　identifying　the　stiffness　weakness　of　cantilever　structures.　To　verify　the　proposed　method,　the　element　stiffness　weakening　is　used　to　simulate　the　weak　parts　regarding　stiffness.　Several　scenarios,　with　single　and　multiple　weakness　points,　including　various　noise　levels,　are　studied,　using　finite　element　simulations.　Next,　a　novel　method,　to　measure　the　accuracy　of　the　algorithm　and　quantify　the　weakness　level,　under　noise　conditions,　is　proposed.　The　advantage　of　this　method,　compared　to　the　ones　based　on　Flexibility　Difference　Method,　is　the　higher　identification　accuracy　under　noise　interference.　Finally,　the　cantilever　beam　with　elastic　support　is　experimentally　studied.　The　natural　frequencies　and　modal　shapes　are　obtained,　according　to　the　singular　value　decomposition　method,　to　establish　the　flexibility　matrix.　In　addition,　using　only　the　lowest　three　modes,　a　series　of　numerical　examples　and　experiments　are　provided,　to　illustrate　the　validity　and　the　considerable　practical　engineering　value　of　the　method.