Micro-mixer　has　great　application　potentials　in　many　fields　such　as　material　synthesis,　pharmaceutical　preparation　and　biochemical　detection　due　to　its　advantages　of　saving　reagents,　higher　mixing　index　and　easy　integration.　In　order　to　further　improve　the　mixing　performance,　to　ensure　the　safety　of　the　mixing　process　and　the　accuracy　of　the　biochemical　reaction　results,　a　new　square　wave　micro-mixer　with　extended　cavity　was　designed.　Under　the　premise　of　considering　the　mixing　index　and　pressure　drop,　the　effects　of　the　width　of　slit,　the　length　of　slit,　and　the　height　of　extended　cavity　on　the　mixing　performance　of　micro-mixer　were　analyzed　by　experiment　and　simulation.　The　optimal　structural　parameters　were　achieved　under　different　Reynolds　number　(Re).　Compared　with　the　square-wave　micro-mixer,　the　mixing　strength　of　the　square-wave　micro-mixer　with　the　extended　cavity　is　higher　when　Re　≥　5.　Moreover,　the　gap　of　mixing　index　between　two　square-wave　micro-mixer　reaches　maximum,　up　to　12%,　at　Re　=　20.　Under　the　same　Reynolds　number,　the　pressure　drop　of　the　square-wave　micro-mixer　with　the　extended　cavity　is　lower　than　that　of　the　square-wave　micro-mixer.　Meanwhile,　the　analysis　for　the　internal　flow　field　of　the　square-wave　micro-mixer　with　the　extended　cavity　was　carried　out.　It　is　found　that　the　eddy　current　is　introduced　on　the　basis　of　the　laminar　flow　state　of　the　fluid　because　of　the　existence　of　the　extended　cavity　structure,　which　means　the　change　of　the　flow　state　of　the　fluid　in　the　channel　and　the　enhancement　of　convection　effect,　thus　the　mixing　performance　is　further　improved.　©　2018,　Editorial　Office　of　Chinese　Journal　of　Theoretical　and　Applied　Mechanics.　All　right　reserved.