The　pipe-in-pipe　structures　are　widely　applied　for　deep-sea　exploration　of　crude　oil　and　natural　gas.　This　study　established　a　mechanical　model　of　deep-sea　pipe-in-pipe　for　S-lay　operation.　In　the　model,　a　simplified　equivalent　calculation　method　was　presented,　and　the　clashing　contacts　between　the　pipeline　and　stinger　rollers　as　well　as　pipe-seabed　interactions　were　fully　taken　into　account.　The　static　results　obtained　by　numerical　calculation　were　in　good　agreement　with　the　analytical　solution.　The　dynamic　response　analysis　were　further　conducted　under　the　combined　action　of　random　waves,　ocean　currents　and　pipelaying　vessel　motions.　The　response　results　of　equivalent　pipe　as　well　as　outer　pipe　and　inner　pipe　in　the　pipe-in-pipe　structure　were　obtained,　and　the　influences　of　outer　pipe　and　inner　pipe　diameter-to-thickness　ratios　on　the　response　amplitude　were　investigated.　The　results　indicated　that　the　equivalent　model　of　the　pipe-in-pipe　structure　could　effectively　simulate　the　pipeline　dynamic　behaviors,　and　that　the　tension,　bending　moment,　stress　and　strain　responses　of　outer　pipe　were　much　larger　than　those　of　inner　pipe.　The　corresponding　response　amplitudes　were　greatly　influenced　by　the　outer　pipe　and　inner　pipe　diameter-to-thickness　ratios.　These　results　could　provide　a　theoretical　basis　for　the　initial　design　of　deep-sea　laying　pipe-in-pipe　structures.　©　2020,　Editorial　Board　of　Journal　of　Ship　Mechanics.　All　right　reserved.