Based　on　an　Eulerian/Eulerian　numerical　model　for　the　steam　and　liquid　two-phase　flow,　the　self-excited　oscillation　flow　of　wet　steam　in　a　convergent-divergent　nozzle　was　numerically　investigated,　and　the　mechanism　of　the　phenomenon　was　analyzed.　From　the　aerodynamics　point　of　view,　the　transitional　process　from　steady　to　unsteady　flow　was　described.　Based　on　the　mechanism　of　different　oscillation　flows　and　shock　wave　kinematics,　the　flow　characteristic　was　explained　that　with　the　increased　inlet　supercooling　and　constant　inlet　pressure,　the　self-excited　frequency　of　oscillating　flow　decreases　first　and　then　increases.　Mechanisms　of　secondary　condensation　and　shock　wave　motion　were　analyzed.　In　the　convergent　section　of　the　nozzle,　condensation　may　also　cause　self-excited　oscillation.