At　present　few　studies　have　considered　the　mechanical　properties　of　welded　ultra-high　strength　(UHSS)　steel　after　fire.　In　this　paper,　two　kinds　of　ultra-high-strength　steels　Q890　and　Q960　are　selected　as　research　object,　and　the　GMAW　welding　method　is　used　to　connect　them.　An　experimental　study　is　conducted　to　reveal　the　mechanical　properties　of　welded　UHSS　specimens　after　exposure　to　different　high　temperatures.　The　butt-welded　UHSS　and　the　corresponding　base　metal　specimens　are　firstly　heated　from　200　°C　to　1000　°C.　After　natural　cooling　to　room　temperature,　the　specimens　are　then　subjected　to　a　tensile　test,　and　the　stress-strain　curves　and　different　fracture　modes　are　obtained.　The　results　shown　that　temperature,　HAZ　and　softening　effect　have　an　impact　on　the　fracture　mode　and　mechanical　properties　of　butt-welded　UHSS.　Secondly,　the　variation　law　of　elastic　modulus　of　butt-welded　UHSS　with　increasing　temperature　is　similar　to　that　of　UHSS　base　metal.　However,　as　the　temperature　increases,　the　corresponding　weakening　rates　of　yield　strength　and　ultimate　strength　are　significantly　lower　than　those　of　the　base　material.　Finally,　based　on　the　experimental　results,　prediction　equations　for　the　reduction　factors　of　mechanical　properties　indicators　of　UHSS　(base　metal　and　welded)　and　HSS　after　fire　exposure　are　proposed,　and　a　comparative　analysis　is　conducted　with　the　existing　methods.　The　results　shown　that　the　predictive　equations　proposed　in　this　paper　can　more　accurately　calculate　the　mechanical　properties　reduction　factors　of　UHSS　(base　metal　and　welded)　and　HSS　after　fire.　©　2024