Use　of　immobilized　enzymatic　membrane　reactors　(EMR)　is　an　effective　strategy　to　enhance　oligodextran　production　efficiency.　Nevertheless,　the　reaction-separation　coupling　mechanism　has　not　been　fully　understood.　In　this　study,　we　developed　a　model　to　disentangle　the　mechanism　and　interaction　behind　the　reaction　and　separation　phenomena　in　the　immobilized　EMR　to　eventually　evaluate　process　performance　for　optimal　oligodextran　production.　The　simulation　results　show　that　in　the　immobilized　EMR　applying　substrate　feeding　mode　and　high　operating　pressure　enhances　convective　and　diffusive　flows　through　membrane,　leading　to　a　high　transport　rate　of　product.　The　optimal　substrate　concentration　and　enzyme　loading　amount　need　to　match　the　selectivity　and　permeability　of　the　membrane,　in　order　to　ensure　rapid　removal　of　the　target　oligodextrans.　Therefore,　the　membranes　with　high　permselectivity　are　required　for　obtaining　desirable　production　efficiency　and　product　quality.　Although　immobilized　EMR　has　higher　reaction　rate　at　the　beginning　because　of　higher　local　enzyme　concentration　in　the　reaction　zone,　the　EMR　with　free　enzymes　could　outperform　the　immobilized　EMR　by　using　higher　enzyme　concentration　due　to　its　larger　reaction　area　as　the　reaction　continues.　Our　study　provides　deep　insights　into　the　reaction-separation　coupling　mechanism　and　EMR　performance　enhancement.　©　2021　The　Authors