Liquid-liquid　slug　flow　has　great　application　potential　in　many　fields.　The　effective　intensification　of　mass　transfer　is　an　important　impetus　for　its　wider　application.　However,　it　still　lacks　a　method　which　can　intensify　its　mass　transfer　at　low　flow-rate　without　slug　breakup.　Hereon,　the　periodic　expansion　microchannel　is　used　to　induce　the　interface　deformation　of　the　droplets　in　slug　flow　to　strengthen　mass　transfer.　Concentration　field　obtained　by　experimental　analysis,　and　velocity　field　obtained　through　numerical　simulation　are　used　to　investigate　the　influence　of　expansion　structure　on　the　mass　transfer　mechanism　of　liquid–liquid　slug　flow.　The　regulating　mechanism　of　mass　transfer　is　disclosed　by　investigating　the　effect　of　operating　conditions　on　the　volumetric　mass　transfer　coefficient　kLa　and　intensification　factor　E.　Optimized　E　can　reach　up　to　1.5.　Introducing　expansion　structures　could　intensify　the　mass　transfer　of　slug　flow　in　a　microchannel　with　low　energy　consumption.　©　2023　Elsevier　Ltd