A572　Gr.50　thick　plates　are　commonly　used　in　large-scale　plate　girder　fabrication.　The　welding　cracks　are　likely　contained　in　the　butt　weld　of　A572　Gr.50　thick　plates.　To　evaluate　the　safety　of　welding　defects　of　the　in-service　large-scale　plate　girders　in　boiler　steel　structures,　A572　Gr.50　thick　plate　butt-welded　joints　were　fabricated　to　reflect　the　similar　material　properties　of　the　butt-welded　lower　flange　plate　of　large-scale　plate　girders.　Uniaxial　tensile　tests,　Charpy　V-notch　tests　and　three-point　bending　tests　were　conducted　for　the　base　metal,　heat-affected　zone　and　the　butt　weld　of　this　simulated　welding　joint　respectively.　Combined　with　the　finite　element　analysis,　the　fracture　properties　of　A572　Gr.50　thick　plate　butt-welded　joints　were　investigated.　Results　indicated　that　materials　from　base　metal,　butt　weld　and　heat-affected　zone　showed　good　mechanical　properties,　satisfying　the　relevant　specification　requirements,　and　possessed　good　capability　of　plastic　deformation.　Charpy　impact　energy　reduced　with　the　decrease　in　temperature.　In　addition,　the　base　metal,　weld　and　heat　affected　zone　had　good　impact　toughness.　With　respect　to　the　low-temperature　brittleness,　the　brittle　resistant　ability　of　the　base　metal　was　the　best　while　that　of　weld　was　the　worst.　Compared　with　base　metal　and　heat-affected　zone　steel,　fracture　toughness　of　molten　zone　was　still　the　worse.　To　summarize,　more　attention　should　be　paid　to　the　butt　weld　zone　of　the　thick-plate　welding　joints　for　their　safety　assessment.　Finite　element　software　packages　can　be　used　to　analyze　the　fracture　properties　of　steel　structural　members　with　cracks　under　working　conditions　to　ensure　structural　safety.　©　2018,　Engineering　Mechanics　Press.　All　right　reserved.