Determining　the　weak　parts　of　a　structure　is　one　of　the　key　issues　in　the　field　of　machine　tool　stiffness　improvement.　However,　studies　show　that　overcoming　the　static　deformation　with　acquisition　difficulty　is　a　complex　problem　in　practical　structures.　This　study　considers　the　machine　tool　cantilever　structure,　as　a　cantilever　beam　and　bar　structure,　where　the　objective　is　to　propose　a　weakness　index,　to　identify　the　weak　part,　using　system　reconstruction　to　extract　the　measured　static　deformation　data　and　the　fitting　data.　Stiffness　reduction　is　used　to　simulate　weak　parts,　while　the　effectiveness　of　the　method　is　evaluated,　in　the　case　of　various　weakness　values　and　of　different　noise　levels,　using　the　finite　element　simulation　approach.　The　validity　of　the　proposed　method　is　illustrated　through　comparison　of　the　theoretical　results　to　the　experimental　ones,　using　the　cantilever　structure　of　a　test　machine　tool.　The　research　content　provides　some　means　of　improving　the　machining　accuracy　of　machine　tools.