Earthquake-induced　fractures　in　steel　structures　are　characterised　by　high-strain　low-cycle　conditions.　In　order　to　investigate　the　ultra-low　cyclic　fatigue　fracture　of　steel　welded　joints　under　earthquakes,　two　most　commonly　used　structural　steels　(Q235B　and　Q345B)　and　the　corresponding　welds　were　studied　by　experiments　and　numerical　analysis　in　this　paper.　Specimens　were　extracted　from　the　base　material,　the　weld　metal　and　the　heat　affected　zone　to　investigate　the　behaviour　in　different　parts　of　the　welded　joint.　Eighteen　smooth　round　bars　were　tested　under　large　strain　amplitudes,　the　hysteretic　properties,　damage　degradation　characteristics　and　failure　process　were　analyzed.　Constitutive　model　named　Chaboche　model　was　calibrated　to　describe　the　cyclic　hardening　behaviour　of　these　materials.　Seventy-two　notched　round　bars　with　three　different　notch　sizes　and　two　loading　protocols　were　tested　to　study　the　fracture　behaviour　of　different　materials　at　different　stress　triaxialities　and　different　strain　amplitudes.　Two　micromechanical　fracture　models:　cyclic　void　growth　model　and　degraded　significant　plastic　strain　model　were　calibrated　based　on　the　test　results.　The　micromechanical　models　and　Chaboche　model　were　incorporated　into　numerical　simulations　by　software　ABAQUS　with　subroutine　VUMAT　to　predict　the　materials　fracture.　The　results　show　that　the　failure　process　under　cyclic　loads　is　opposite　to　that　of　monotone　loads.　The　dissipation　capacity　of　Q345B　is　superior　to　that　of　Q235B.　The　fracture　resistance　deteriorate　more　in　the　weld　zone　under　the　same　loading　conditions.　The　validated　models　can　be　used　to　effectively　and　accurately　evaluate　the　fracture　in　steel　welded　connections　under　ULCF　conditions.