To　research　the　mechanical　properties　and　low　cycle　fatigue　performance　of　Q355B　structural　steel　and　Q355GNHB　weathering　steel,　this　study　performs　monotonic　and　cyclic　tests　on　the　base　metal,　welded　metal　and　heat-affected　zone　(HAZ)　of　Q355GNHB　welded　connections　and　Q355B.　The　monotone　and　cyclic　constitutive　model　parameters　are　calibrated　using　the　true　stress-strain　relationship　and　the　Chaboche　hybrid　strengthening　model,　and　their　accuracy　is　verified　by　ABAQUS.　To　streamline　the　calibration　process,　a　simplified　calibration　method　is　established　for　the　Chaboche　combined　hardening　model.　Compared　to　the　traditional　calibration　method,　the　proposed　method　uses　the　stress-strain　data　obtained　from　the　monotonic　tests.　The　applicability　of　the　calibrated　model　is　verified　by　simulating　the　tests　via　ABAQUS.　The　research　results　show　that　both　Q355B　structural　steel　and　Q355GNHB　weathering　steel　have　excellent　bearing　capacity　and　plastic　deformation　capacity.　The　Q355GNHB　weathering　steel　has　better　ductility,　while　the　Q355B　structural　steel　has　higher　bearing　capacity　and　stronger　strength　reserve　capacity.　The　strength　of　Q355GNHB　varies　according　to　different　zones:　the　yield　strength　of　the　base　metal　material　is　the　lowest,　while　the　tensile　strength　and　the　plastic　deformation　capacity　of　the　welded　metal　material　is　the　lowest.　Besides,　the　Chaboche　combined　hardening　model　and　simplified　combined　hardening　model　can　capture　the　cyclic　fatigue　performance　of　Q355B　and　Q355GNHB　steel.　The　research　results　provide　effective　data　support　for　the　in-depth　study　of　the　constitutive　relations　and　mechanical　properties　of　weathering　steel　materials　and　the　accurate　simulation　of　the　properties　of　weathering　steel　structures.