Although　it　was　experimentally　verified　the　microchannels　with　periodic　expansion　structures　can　effectively　intensify　the　mass　transfer　of　the　slug　flow,　little　research　has　focused　on　their　geometric　design.　Hereon,　a　numerical　simulation　is　conducted　to　evaluate　the　impact　of　microchannel　geometry　on　the　mass　transfer　of　slug　flow.　The　square　expansion　unit　performs　better　than　the　triangular　and　circular　ones.　With　the　increase　of　the　length,　width,　and　number　of　the　expansion　units,　and　the　width　of　the　constriction　between　the　adjacent　expansions,　the　mass　transfer　performance　shows　a　non-monotonic　variation,　that　is　an　initial　increase　followed　by　a　decrease.　The　underlying　reasons　are　analyzed　by　evaluating　the　mixing　inside　and　outside　the　droplet　and　the　effective　mass　transfer　area.　At　the　preliminarily　optimized　geometry,　a　novel　mass　transfer　path,　in　which　not　only　the　droplet　caps　but　also　the　droplet　body　becomes　the　effective　mass　transfer　area,　is　observed.