To　establish　the　fatigue　crack　initiation　analysis　method　for　U　rib-cover　plate　welded　joints　of　steel　bridge　decks,　the　conversion　of　cohesive　parameters　in　the　mixed　loading　mode　was　considered　based　on　the　Roe-Siegmund　cyclic　cohesive　zone　model,　the　secondary　development　of　ABAQUS　was　carried　out,　and　the　VUMAT　subroutine　reflecting　the　cumulative　fatigue　damage　was　formed.　The　cohesive　parameters　of　the　material　in　the　welding　zone　corresponding　to　the　Q345　steel　were　obtained　from　the　experimental　data.　Based　on　the　Voronoi　diagram　method　and　the　grain　microstructure　and　mechanical　characteristics　of　the　welding　zone,　the　microscopic　grain　structure　at　the　U　rib-cover　plate　weld　toe　was　established.　In　addition,　combined　with　the　macroscopic　2D　plane　strain　model,　the　multi-scale　fatigue　crack　initiation　was　simulated.　Combined　with　the　equivalent　structural　stress　method　and　the　crack　propagation　theory　of　linear　elastic　fracture　mechanics,　the　accumulated　cohesive　lengths　under　different　stress　levels　were　backpropagatcd　considering　the　initial　defect　morphology　and　critical　criterion　of　fatigue　fracture,　and　then　the　calculation　method　for　the　crack　initiation　life　was　obtained.　Analysis　results　show　that　when　using　the　proposed　method　to　simulate　the　crack　initiation　behavior　at　the　weld　toe　of　U　rib-cover　plate　welded　joint,　the　cracks　initiate　at　the　weld　toe　and　propagate　perpendicular　to　the　top　plate　surface,　forming　a　transcrystallinc　fracture　mode.　The　stress　distribution　of　the　microscopic　grain　structure　changes　with　crack　initiation　and　short　crack　propagations,　and　as　the　microscopic　grain　structure　distribution　and　mechanical　characteristics　change　randomly,　the　details　of　short　crack　propagation　paths　and　critical　cycle　numbers　in　the　simulation　results　are　not　the　same.　The　backpropagatcd　accumulated　cohesive　length　varies　with　the　initial　defect　morphology　ratio,　the　critical　depth　of　long　crack　propagation,　the　distribution　and　mechanical　characteristics　of　microscopic　grain　structure,　and　the　stress　amplitude.　The　fitted　curves　of　accumulated　cohesive　length　and　equivalent　structural　stress　amplitude　obtained　by　considering　the　above　factors　arc　capable　of　obtaining　the　corresponding　crack　initiation　lifes.　Therefore,　the　established　multi-scale　fatigue　crack　initiation　simulation　analysis　method　can　provide　a　new　solution　to　obtain　the　fatigue　crack　initiation　lifes　of　steel　bridge　decks.　3　tabs,　17　figs,　31　refs.　©　2024　Chang＇an　University.　All　rights　reserved.