Understanding　the　interactions　between　hydrogen　and　material　integrity　in　polycrystalline　alpha-Fe　is　essential　for　advancing　the　reliability　of　critical　infrastructure　and　energy　systems.　In　this　study,　molecular　dynamics　simulations　were　implemented　to　pinpoint　the　crack　propagation　behaviour　and　mechanism　in　polycrystalline　alpha-Fe　under　various　hydrogen　concentrations　and　temperatures.　The　results　show　that　a　phase　transition　from　body-centred　cubic　to　face-centred　cubic　structure　first　occurs　at　the　crack　tip,　followed　by　grain　boundary-mediated　plasticity　activities　at　room　temperature　devoid　of　hydrogen.　A　limited　amount　of　hydrogen　atoms　(H/Fe　atomic　ratio＜1%)　induces　twinning　emission　from　the　tip,　and　increasing　temperature　further　enhances　dislocation　plasticity　as　a　consequence　of　decreased　unstable　stacking　fault　energy,　thereby　leading　to　the　blunting　of　the　crack　tip.　At　high　hydrogen　concentrations　(H/Fe　atomic　ratio＞1%),　the　formed　hydrides　ahead　of　the　crack　tip　suppress　the　phase　transition,　and　concurrently　temperature-enhanced　dislocation　plasticity　disappears.　As　a　consequence,　the　crack　propagation　proceeds　via　grain　boundary　cavity　nucleation　and　growth,　and　ultimately　evolves　into　intergranular　fracture.　These　findings　provide　an　atomistic-level　explanation　for　temperature-dependent　hydrogen-crack　interaction　mechanisms,　and　reveal　a　transition　in　the　fracture　mode　from　ductile　transgranular　to　intergranular　failure　associated　with　locally　high　hydrogen　concentrations　found　in　the　experiments.