Rapid　development　of　new　energy　vehicle　desires　more　advanced　lightweight　design;　such　need　makes　the　use　of　more　Al　continuously　attractive　to　automotive　industry.　Since　welding　is　the　most　commonly　used　method　to　build　a　car,　the　quality　of　Al/steel　dissimilar　joint　becomes　a　key　factor　influencing　the　vehicle　reliability.　There　is　a　common　sense　that　Al/steel　dissimilar　joint　would　fracture　either　along　Al/steel　interface　or　Al　heat-affected　zone　(HAZ),　but　the　mechanism　of　such　fracture　mode　variation　is　lack　of　experimental　support.　And　as　the　joint　interface　is　inevitably　composed　of　Fe2Al5　and　Fe4Al13　intermetallic　layers,　the　detailed　fracture　procedure　is　usually　too　fast　to　be　captured　in　situ　and　therefore　rarely　reported.　In　the　present　work,　the　fracture　mode　of　Al/steel　dissimilar　overlap　joints　made　using　variable　polarity　cold　metal　transfer-based　arc　brazing　is　investigated　using　high-speed　camera　under　special　instrument　setup.　And　by　combining　the　high-speed　camera　results　with　interface　material　characterization　results,　the　mechanism　of　fracture　mode　variation　in　Al/steel　dissimilar　overlap　joint　under　axial　tensile　force　is　clarified.　It　is　found　that　along　with　the　increase　in　arc　welding　heat　input,　the　fracture　propagation　path　would　experience　a　variation　in　＂Al/steel　interface　-＞　Al　HAZ.＂　And　such　variation　is　practically　induced　by　the　synergistic　effect　of　both　overlap　joint　stress　concentration　and　Al/steel　interface　strength,　which　is　subsequently　confirmed　by　numerical　model　analysis　and　a　specially　designed　interface　shear　strength　test.