Aiming　at　the　need　of　safe　operation　of　power　plant,　the　nonlinear　ultrasonic　detection　method　of　thermal　damage　of　high　temperature　pipeline　by　collinear　mixing　is　studied.　The　excitation　conditions　of　significant　mixing　effect　were　determined　by　frequency　sweep　experiment,　and　nonlinear　ultrasonic　testing　experiments　were　carried　out　on　four　Super304H　pipes　with　different　thermal　damage　degrees.　The　detection　signals　were　analyzed　by　bispectrum　to　investigate　the　distribution　of　bispectrum　of　sum　frequency　component　in　phase　interval,　and　the　phase　distribution　range　of　nonlinear　response　caused　by　thermal　damage　was　determined.　The　results　show　that　the　nonlinear　acoustic　coefficients　extracted　in　turn　have　a　good　correlation　with　the　thermal　damage　degree　of　the　specimen,　which　can　be　used　to　characterize　the　thermal　damage　of　the　pipeline.　The　research　work　provides　a　feasible　scheme　for　thermal　damage　detection　of　high　temperature　pipelines　in　power　plants.　©　2024　Atomic　Energy　Press.　All　rights　reserved.