By　applying　optimal　in-situ　post-weld　heat　treatment　(PWHT),　in-situ　austenitization　was　achieved　in　the　fusion　line　(FL)　of　ultra-high　strength　steel　(2　GPa　steel,　tensile　strength　exceeding　2000　MPa)　resistance　spot　welding　(RSW)　joint,　and　mechanical　properties　of　the　joint　were　significantly　improved.　In　the　fusion　line　of　7　kA　weldment　without　in-situ　post-weld　heat　treatment,　the　presence　of　a　C-poor　zone　led　to　the　formation　of　fine-lath　gamma-austenite　and　a　decrease　in　dislocation　density,　resulting　in　the　＂hard-soft-hard＂　sandwich　structure.　The　cracks　of　the　7　kA　weldment　propagated　along　the　fusion　line.　The　maximum　force　of　7　kA　weldment　was　only　12.8　kN.　The　optimal　in-situ　post-weld　heat　treatment　parameter　(6　kA　+　2　s)　caused　the　re-partitioning　of　the　C　element　and　the　formation　of　full　martensite　in　the　original　fusion　line,　eliminating　the　＂hard-soft-hard＂　sandwich　structure.　Consequently,　the　crack　in　7　kA-(6　kA　+　2　s)　weldment　no　longer　propagated　along　the　fusion　line.　The　maximum　force　of　7　kA-(6　kA　+　2　s)　weldment　reached　22.2　kN.　The　efficient,　accurate　and　optimal　post-weld　heat　treatment　process　can　eliminate　element　segregation　and　change　the　microstructure　in　the　fusion　line　of　ultra-high　strength　steels　resistance　spot　welding　joints,　thereby　altering　the　crack　propagation　path　and　significantly　enhancing　the　mechanical　properties　of　the　joints.