The　pipe-in-pipe　(PIP)　system,　with　good　structural　resistance　and　favourable　thermal　insulation　capacity,　has　been　extensively　applied　in　oil　and　natural　gas　exploitation　in　deep　waters.　In　the　present　paper,　a　simplified　equivalent　numerical　model　of　the　PIP　system　for　deepwater　J-lay　operation　was　developed　to　evaluate　the　dynamic　response　of　the　outer　pipe　and　the　inner　pipe　under　the　combined　hydrodynamic　load　and　pipelay　vessel　motion　by　the　software　OrcaFlex.　The　comparison　of　mechanical　responses　between　the　present　equivalent　model　and　other　available　PIP　models　was　performed　to　verify　its　reasonability.　Considering　vessel　motion,　pipe-soil　interaction,　wave　and　current,　the　dynamic　behaviour　of　the　outer　and　inner　pipes　was　evaluated　on　aspects　of　the　bending　moment,　effective　tension,　equivalent　stress　and　strain.　After　that,　the　influences　of　key　geometric　parameters　on　the　dynamic　behaviour　of　PIP　systems　were　systematically　studied,　including　the　diameter-to-thickness　ratios　of　the　outer　pipe　and　inner　pipe　as　well　as　the　core　thickness.　The　findings　would　provide　good　guidance　for　the　structural　design　and　the　installation　analysis　of　PIP　systems　using　the　deepwater　J-lay　operation.