For　the　influence　of　stress　ratio　on　fatigue　life,　fatigue　tests　of　TC18　titanium　alloy　produced　by　build-up　welding　(TC18　by　BW)　samples　were　carried　out　under　three　stress　ratios　(R=0.5,　R=　0.06,　R=-1),　to　draw　three　fatigue　limits　and　6　S-N　curves　of　＇stress　amplitude　life　model＇　and　＇three-parameter　model＇.　Based　on　the　integral　relationship　of　the　crack　growth　rate　and　fatigue　life　and　considering　both　of　mathematical　models　of　fatigue　life,　a　systematic　investigation　of　the　relationship　between　stress　ratio　(R)　and　fatigue　life　curve　(S-N)　was　performed　to　build　the　fatigue　life　mathematical　model　(RS-N).　According　to　the　modified　formula　proposed　in　this　paper,　the　establishment　of　two　R-S-N　mathematical　models,　applicable　for　TC18　by　BW　materials.　The　results　show　that　＇stress　amplitude＇　model　can　accurately　predict　moderate　fatigue　life,　and　＇three-parameter　fatigue＇　model　is　more　suitable　for　the　prediction　of　long-life　area.　The　predictive　value　of　two　proposed　R-S-N　mathematical　models　are　in　better　agreement　with　the　experimental　values,　they　can　accurately　predict　the　fatigue　curve　under　any　stress　ratio　in　engineering.　©　All　right　reserved.