An　instability　theoretical　model　for　multi-channel　system　had　been　developed　by　building　one-dimensional　homogeneous　model　and　adopting　nonlinear　analysis　for　two-phase　flow　in　vertical　single　pipe,　supplied　by　Clausse　and　Lahey.　With　the　same　parameters　as　the　experiment,　two-phase　flow　density　wave　oscillation　in　parallel　inclined　inner　ribbed　pipes　was　analyzed　and　solved　in　this　paper.　The　variation　of　the　working　fluid　inlet　velocity　perturbation　with　time　was　found　using　the　time　domain　method.　The　criteria　of　density　wave　oscillation　were　defined　according　to　the　convergence　or　divergence　of　the　velocity.　The　pulse　cycle　of　calculated　results　was　mostly　between　5　and　16　seconds,　which　was　quite　similar　to　the　data　between　7　and　19　seconds　in　the　experiment.　It　is　shown　that　the　calculated　results　have　a　good　agreement　with　the　experimental　data.　Therefore,　the　method　can　be　used　to　determine　whether　the　incidence　of　density　wave　instabilities.　It　can　also　provide　a　reference　for　the　safety　parameters　of　the　water　wall　in　the　supercritical　pressure　boiler.