Abstract: Drilling riser is the casing from offshore drilling plateform to subbottom,it plays an important role in offshore completion.In deep sea,with the increasing of riser length above mudline,the current vortext-induced effect to drilling riser fatigue strength is adding,so the riser life is decreased.The research could offer science basis to riser size and material choosing.In this paper,the mechanical analyses of the riser had been made,it simplifies the mechanical model as the low fixed and the top simply supported beam.Based on liquid damping and vortex-induced force theoretic model,calculated it’s fatigue strength.Interacted with oceanic condition of an area from South China Sea,fatigue strength and service life of offshore drilling riser were analysed and checked,eventually,the reasonable size was provided.From the research:with the depth increasing,current vortext-induced effect will gradually strengthen,and the influence to fatigue strength aggravates.