The　mechanical　properties　of　high-strength　steel　(HSS)　hollow　section　joints,　including　those　after　fire,　have　been　widely　investigated　recently.　However,　for　the　post-fire　performance　of　high-strength　steel　(HSS)　hollow　section　joints,　air-cooling　(CIA)　methods　have　been　mainly　considered,　and　there　is　currently　no　research　on　watercooling　methods　(CIW).　The　post-fire　residual　strength　and　calculation　methods　of　rectangular　high-strength　steel　X　(RHS-X)　joints　under　water　cooling　conditions　is　studied　through　numerical　simulation　methods　in　this　paper.　Firstly,　a　refined　numerical　model　of　RHS-X　joint　is　established,　and　validated　using　experimental　results　of　RHS-X　joints　under　air-cooling　conditions　(CIA),　Then,　based　on　the　various　post-fire　mechanical　properties　predicted　equations　of　HSS　under　water-cooling　conditions,　a　simplified　water-cooling　ideal　elastoplastic　residual　constitutive　model　of　HSS　is　established,　and　a　comparative　analysis　of　the　post-fire　residual　strength　of　RHS-X　joints　under　different　cooling　methods　is　carried　out　through　numerical　simulation;　Finally,　parameter　analysis　is　conducted　and　a　corresponding　calculation　method　for　post-fire　residual　strength　of　RHS-X　joints　under　CIW　is　proposed,　and　compared　with　the　calculation　methods　provided　by　PR　EN　1993-1-8　and　GB　50017-2017　and　methods　proposed　by　different　researchers.　The　results　show　that　the　residual　properties　of　high-strength　steel　after　fire　exposure　are　different　under　air-cooling　and　water-cooling　conditions,　mainly　due　to　the　redistribution　of　cementite　after　fire,　which　will　inevitably　affect　the　residual　strength　of　the　structures,　therefore,　the　post-fire　mechanical　properties　of　RHS-X　joints　are　different　when　considering　different　cooling　methods,　the　model　proposed　in　this　paper　can　more　reasonably　predict　the　residual　strength　of　RHS-X　under　CIW.