The　effect　of　crack　propagation　on　the　changes　of　natural　frequency　was　studied　through　simulation　of　crack　propagation　process　by　dispatching　element　nodes　one　by　one　in　a　finite　element　model.　Based　on　the　damage　mechanics　principle,　the　projection　area　of　crack　on　cross-section　was　used　to　define　the　damage,　and　to　establish　the　fatigue　life　prediction　model　with　stress　amplitude　and　mean　stress.　The　fatigue　life　prediction　by　this　model　showed　good　agreement　with　experimental　results.　The　proposed　life　prediction　model　can　effectively　predict　the　fatigue　life　of　spot　welded　joints.