Flow　instability　of　Supercritical　Water-cooled　Reactor　CSR1000　was　studied　and　mathematics　model　of　core　in　supercritical　water-cooled　reactor　CSR　1000　was　established.　A　code　named　FREDO-CSR1000(Frequency　domain　analysis　of　CSR1000)　and　a　code　named　TIMDO(Time-Domain　Method)　have　been　developed　to　analyze　the　flow　instability　of　Supercritical　Water-cooled　Reactor　CSR1000　after　the　codes　was　verified.　The　results　show　that　the　shape　of　stability　map　obtained　by　the　two　different　methods　are　very　similar,　both　of　which　are　divided　into　two　regions,　respectively　corresponding　to　two　types　of　flow　instability,　namely　the　flow　drift　and　the　density　wave　oscillation　instability.　Besides,　it　is　also　found　that　the　operation　points　of　CSR1000　calculated　by　the　frequency　domain　method　and　time　domain　method　are　both　in　the　safety　operation　region.,　which　are　far　away　from　the　unstable　region.