Characteristics　of　gas　flow　in　a　2-D　Laval　micro-nozzle　with　37m　throat-width　were　studied　through　numerical　simulation　based　on　continuum　theory　in　this　paper.　Influences　of　pressure　ratio　on　the　configuration　of　the　flow　field　were　investigated　in　detail.　The　formation　mechanism　and　requirement　of　supersonic　annular　phenomenon,　whose　essence　is　＇Mach　disk＇　formed　in　micro-nozzle　under　over-dilation　condition,　were　analyzed　in　micro-scale　by　shock　wave　theory.　While　the　exit　pressure　remained　unchanged,　the　possible　existent　range　of　supersonic　annular　phenomenon　was　judged,　and　the　influence　of　pressure　ratio　and　shock　wave　on　total　pressure　drop　in　the　micro-nozzle　was　discussed.