For　the　arbitrary　analysis　of　an　accident　and　to　identify　the　true　causes　of　rupture　of　the　tie　rod,　a　series　of　studies,　from　theories　to　experiments　on　the　bended　broken　tie　rod,　are　conducted.　A　mechanical　model　is　established　and　the　stability　of　the　defective　tie　rod　is　simulated　based　on　ANSYS　software.　Meanwhile,　the　process　of　the　accident　is　simulated　considering　the　effect　on　destabilization　of　different　vehicle　speeds　and　directions　of　the　impact.　Simultaneously,　macro　graphic　tests,　chemical　composition　analysis,　microstructure　analysis　and　SEM　analysis　of　the　fracture　are　implemented.　The　results　show　that:　1)　the　toughness　of　the　tie　rod　is　at　a　normal　level,　but　there　are　premature　flaws.　One　quarter　of　the　fracture　surface　has　cracked　before　the　accident.　However,　the　flaw　has　no　direct　relationship　with　this　incident.　The　direct　cause　of　the　accident　is　the　dynamic　instability　caused　by　the　large　deformation　of　the　car　under　impact　loading.　2)　The　safety　factor　of　the　tie　rod　declines　greatly　due　to　the　premature　flaws　and　the　result　of　numerical　simulation　shows　that　premature　flaw　is　the　vital　factor　of　structure　stability　on　the　basis　of　the　comparison　of　critical　loads　between　the　accident　tie　rod　and　a　normal　one.　The　critical　load　decreases　by　51.3%　when　the　initial　defect　reaches　19.54%　of　the　cross-sectional　area,　which　meets　the　standards　of　Theory　of　Koiter.