The　weak　part　of　the　stiffness　of　machine　tool　combined　structure　is　the　key　to　improve　the　stiffness　of　machine　tool.　To　overcome　the　static　deformation　with　difficulty　acquisition,　the　paper　chooses　machine　tool　combined　structure　which　can　be　equivalent　to　one-dimensional　bar　structure,　and　a　weakness　index　(WI)　is　proposed　to　identify　the　weak　part　of　the　stiffness　by　means　of　the　dynamic　hammer　test　method.　Based　on　the　bar　structure　as　a　numerical　example,　the　weak　parts　are　modeled　as　EA　reduction　in　stiffness　while　the　mass　is　maintained　at　a　constant　value.　Thorough　finite　element　(FE)　method　simulations　are　performed　to　assess　the　robustness　and　limitations　of　the　method　in　several　scenarios　with　single　and　multiple　weakness.　On　the　crossbeam　of　gantry　type　machine　tool,　the　sensors　are　used　to　collect　vibration　data,　the　structural　modal　parameters　are　obtained　by　singular　value　decomposition　(SVD)　technique,　and　the　dynamic　characteristics　are　systematically　reconstructed　by　using　modal　state　space　method　to　obtain　stiffness　data　at　zero-frequency.　Then,　the　weak　part　of　the　structural　stiffness　is　identified　by　the　weakness　index.　Finally,　the　comparison　of　FE　simulations　and　experiment　results　are　provided　to　illustrate　the　working　of　the　method.