High-density　polyethylene　(HDPE)　pipelines　are　extensively　utilized　in　energy　transportation　in　the　ocean.　However,　long-term　exposure　to　water　can　alter　the　performance　of　HDPE,　potentially　leading　to　pipeline　accidents.　This　study　focuses　on　simulating　the　aging　characteristics　of　PE100　polyethylene　pipeline　butt-fusion　welded　joints　(B-FWJs)　in　water　using　hydrothermal　accelerated　aging　experiments　at　various　temperature　gradients.　The　performance　of　the　B-FWJ　after　hydrothermal　aging　was　characterized　using　scanning　electron　microscopy　(SEM),　oxidation　induction　time　(OIT),　attenuated　total　reflectance　Fourier　transform　infrared　(ATR　FT-IR)　spectroscopy,　and　mechanical　testing.　Furthermore,　this　study　analyzed　the　performance　characteristics　and　changes　in　the　micro-molecular　chains　of　an　HDPE　B-FWJ　pipeline　following　hydrothermal　aging.　An　investigation　was　conducted　into　the　effects　of　hydrothermal　aging　temperature　and　duration　on　the　physical　and　chemical　characteristics　of　HDPE　B-FWJ,　and　the　aging　mechanism　under　hydrothermal　aging　conditions　was　explored.　The　results　indicate　that　increasing　hydrothermal　aging　temperature　leads　to　a　more　significant　decrease　in　the　mechanical　properties　of　the　B-FWJ.　These　findings　contribute　to　understanding　the　aging　behavior　of　PE100　pipelines　in　the　joint　section　and　offer　insights　to　mitigate　the　risks　associated　with　the　aging　of　and　damage　to　B-FWJ　pipelines　in　the　ocean.