©　2018　Elsevier　Ltd　An　experimental　study　on　the　flow　instabilities　of　ultra-supercritical　water　in　the　water　wall　tubes　of　an　ultra-supercritical　circulating　fluidized　bed　(CFB)　boiler　was　conducted.　The　material　of　the　tubes　is　1Cr18Ni9Ti.　The　scope　of　experimental　parameters　is　as　follow:　pressure　from　23　to　30　MPa,　mass　flow　rate　from　0.05　to　0.2　kg·s−1;　inlet　water　temperature　from　200　to　390　°C,　inlet　pressure　drop　coefficient　from　0　to　5.5,　and　heat　flux　from　0　to　500　kW·m−2.　Variations　of　mass　flow　rate　and　pressure　in　the　parallel　channels　were　recorded　with　the　occurrence　of　flow　instability,　and　oscillation　curves　of　the　wall　temperature　were　obtained.　The　starting　points　and　factors　that　affect　various　types　of　oscillations　were　analyzed　to　derive　a　stability　map.　Results　show　that　pressure　and　mass　flow　rate　display　a　reverse　phase　pulsation　during　oscillation.　With　the　increase　in　heat　flux,　three　regions　of　oscillations,　namely,　region1,　region2　and　region3,　appeared.　The　oscillations　in　region1　and　region2　are　observed　for　the　first　time　under　ultra-supercritical　pressure.　Among　them,　oscillations　in　region1　and　region2　are　system　oscillations,　and　they　have　long　periods　and　large　amplitudes.　Oscillation　in　region3　belongs　to　density　wave　oscillation　(DWO),　which　is　the　intertube　oscillation　and　has　a　short　period　and　small　amplitude.　Increases　in　system　pressure,　inlet　mass　flow　rate,　inlet　pressure　drop　coefficient　or　decreasing　the　inlet　water　temperature　are　conducive　to　system　stability.