Theoretical　analysis　and　experimental　research　on　the　low　cycle　fatigue　damage　of　TC4　titanium　alloy　sheet　and　its　laser　welding　joint　were　conducted.　The　tensile　tests　of　specimens　after　different　fatigue　cycles　show　that　elongation　is　sensitive　to　fatigue　damage,　which　is　used　as　an　effective　physical　parameter　to　characterize　the　low　cycle　fatigue　damage　behavior　of　TC4　titanium　alloy　sheet　and　its　laser　welding　joint.　The　initial　value　of　the　damage　variable　for　base　metal　is　0.037,　which　increases　slowly　with　fatigue　cycles　and　increases　rapidly　when　the　cycles　up　to　90%　of　the　fatigue　life.　Continuum　damage　mechanics　theory　analysis　results　can　be　a　good　explanation　for　the　low　cycle　fatigue　damage　law　of　TC4　base　metal　sheet,　the　theoretical　model　fits　well　with　the　experiment　results.　The　microstructure　of　laser　welding　joint　of　TC4　alloy　is　not　uniform.　There　are　welding　defects,　which　make　the　low　cycle　fatigue　damage　curve　of　the　welding　joint　deviates　from　the　theoretical　model.　The　pores　in　the　welding　joint　make　the　initial　damage　variable　value　change　to　0.4.　As　the　coarse　grains　in　the　welding　joint　center,　the　damage　variable　starts　to　increase　rapidly,　when　the　fatigue　cycles　reaches　50%　of　the　fatigue　life.