A　new　car　in　operation　of　only　8,000　km,　because　of　malfunction,　resulting　in　lost　control　and　rammed　into　the　edge　of　the　road,　and　then　the　basic　vehicle　scrapped.　According　to　the　investigation　of　the　site,　it　was　found　that　the　tie-rod　of　the　car　had　been　broken.　For　the　subjective　analysis　of　the　accident　and　identifying　the　true　causes　of　rupture　of　the　tierod,　a　series　of　studies,　from　the　angle　of　theory　to　experiment　on　the　bended　broken　tie-rod,　were　conducted.　The　mechanical　model　was　established;　the　stability　of　the　defective　tie-rod　was　simulated　based　on　ANSYS　software.　Meanwhile,　the　process　of　the　accident　was　simulated　considering　the　effect　of　destabilization　of　different　vehicle　speed　and　direction　of　the　impact.　Simultaneously,　macro　graphic　test,　chemical　composition　analysis,　microstructure　analysis　and　SEM　analysis　of　the　fracture　were　implemented.　The　results　showed　that:　1)　the　toughness　of　the　tie-rod　is　at　a　normal　level,　but　there　is　some　previous　flaws.　One　quarter　of　the　fracture　surface　has　been　cracked　before　the　accident.　However,　there　is　no　relationship　between　the　flaw　and　this　incident.　The　direct　cause　is　the　dynamic　instability　leading　to　the　large　deformation　of　impact　loading.　2)　The　declining　safety　factor　of　the　tie-rod　greatly　due　to　the　previous　flaws;　the　result　of　numerical　simulation　shows　that　previous　flaw　is　the　vital　factor　of　structure　instability,　on　the　basis　of　the　comparison　of　critical　loads　of　the　accident　tie-rod　and　normal.　The　critical　load　can　decrease　by　51.3%　when　the　initial　defect　increases　19.54%　on　the　cross-sectional　area,　which　meets　the　Theory　of　Koiter.　©　2009　SPIE.