In　order　to　investigate　the　ultra-low　cycle　fatigue　(UCLF)　fracture　in　welded　connections　of　high-strength　steel　(HSS)　under　earthquakes,　this　paper　studies　micromechanical　fracture　models　for　Q460C　steel,　which　has　wide　application　prospects　in　China.　Notched　round　bars　and　smooth　round　bars,　which　were　manufactured　from　base　metal,　heat-affected　zone　(HAZ),　and　weld　metal　of　Q460C　HSS,　were　tested　under　monotonic　loading　and　cyclic　loading,　respectively.　The　fracture　surface　of　tensile　specimens　were　analyzed　by　scanning　electron　microscope　(SEM)　tests.　This　paper　presents　the　results,　including　the　skeleton　curve,　hysteresis　loops,　low　cycle　fatigue　life,　and　characteristic　length　l.　By　comparing　the　experimental　results　with　the　finite-element　analyses,　the　toughness　parameters　of　the　void　growth　model　(VGM),　the　stress-modified　critical　strain　(SMCS)　model,　and　the　cyclic　void　growth　model　(CVGM)　of　Q460C　HSS　and　its　weld　were　calibrated.　Results　indicate　that　the　toughness　parameters　of　various　materials　for　HSS　are　generally　lower　than　that　of　normal　strength　steel.　Also,　in　the　welded　beam-to-column　connections,　the　HAZ　requires　more　attention.　The　results　can　be　used　to　effectively　and　accurately　evaluate　the　UCLF　fracture　in　welded　HSS　connections.