In　this　work,　the　fatigue　crack　growth　(FCG)　behavior　and　fatigue　damage　mechanism　of　the　2.25Cr1Mo0.25V　steel　welded　joint　used　in　hydrogenation　reactors　were　investigated.　The　multi-pass　welding　was　carried　out　to　manufacture　the　welded　joint　using　the　combined　shielded　metal　arc　welding　(SMAW)　and　submerged　automatic　arc　welding　(SAAW)　processes.　The　FCG　behavior　of　different　zones　in　the　welded　joint,　including　the　base　metal　(BM),　the　heataffected　zone　(HAZ)　and　the　weld　metal　(WM),　were　studied　by　compact　tension　tests.　Moreover,　the　acoustic　emission　(AE)　technique　was　used　to　monitor　AE　signals　generated　from　FCG　process　for　further　understanding　FCG　behavior　and　fatigue　mechanisms.　Additionally,　the　microstructures　and　fracture　surfaces　of　different　specimens　were　observed　by　optical　microscopy　(OM)　and　scanning　electron　microscopy　(SEM).　The　results　revealed　that　the　microstructure　of　BM　is　fine　granular　bainite,　while　the　WM　shows　coarser　bainite　grains.　The　HAZ　exhibits　the　most　significant　inhomogeneity　with　large　dispersion　of　grain　size.　FCG　results　showed　that　the　HAZ　exhibits　much　higher　fatigue　crack　growth　rate　(FCGR)　at　low　ΔK　values,　while　the　BM　shows　the　most　superior　fatigue　resistance.　The　AE　technique　is　successful　in　monitoring　and　identifying　damage　evolutions　during　the　FCG　process.　Moreover,　an　enhanced　AE　activity　is　observed　in　FCG　of　the　WM　specimen,　which　is　attributed　to　the　combined　influence　of　the　formation　of　numerous　secondary　cracks　and　coarsegrained　microstructures.　©　2021　by　the　authors.　Licensee　MDPI,　Basel,　Switzerland.