There　is　pressing　need　to　explore　the　fundamental　link　between　the　mixing　efficiency　and　the　entrainment　performance　for　ensuring　an　efficient　working　of　ejector　in　the　associated　energy　systems.　This　study　details　where,　how　and　to　what　extent　the　primary　and　entrained　flows　mix　inside　the　steam　ejector　through　a　novel　ejector　model　with　species　transport.　The　evolution　laws　of　the　mixing　layer　growth,　the　entrainment　performance　and　the　mass　transfer　of　two　streams　are　systematically　discussed　under　various　operational　conditions.　Further,　their　fundament　links　are　clearly　presented　in　a　form　of　functional　relation.　The　key　findings　emerged　from　this　study:　The　mixing　begins　over　a　fluctuating　mixing　layer　since　the　mixing　chamber　entrance.　In　the　situation　of　critical　operational　models,　the　mixing　layer　experiences　a　fluctuating　and　exponential　growth　successively,　but　its　development　is　severely　obstructed　under　the　subcritical　models.　Additionally,　the　entrainment　ratio　and　the　non-mixing　length　ratio　are　highly　linear　correlation　with　the　mass　transfer　ratio.　The　variations　of　operational　parameters　essentially　change　the　relative　mass　transfer　capacity　of　the　entrained　flow.　The　more　easily　the　entrained　flow　mixes　into　the　primary　jet　flow,　a　faster　mixing　layer　growth　and　higher　entrainment　performance　are　to　be　gained.　©　2020　Elsevier　Ltd