Droplet　breakup　in　microconstrictions　is　an　important　phenomenon　in　industrial　applications.　This　work　aimed　to　investigate　the　droplet　breakup　in　the　square　microchannel　with　a　short　square　constriction　to　generate　the　slug　flow,　which　drew　little　attention　before.　Mechanism　analysis　indicated　that　this　breakup　process　included　the　shear-force-dominated,　squeezing-force-dominated,　and　pinch-off　stages.　Nonuniform　daughter　droplets　were　generated　in　the　constriction　with　their　interface　restricted　in　the　horizontal　and　perpendicular　directions　by　the　microchannel　walls.　The　average　relative　deviation　of　the　daughter　droplet　size　was　＜30%,　much　lower　than　that　for　the　breakup　with　the　daughter　droplet　restricted　only　in　one　direction.　An　empirical　equation　with　a　deviation　of　＜20%　was　provided　to　show　the　dependence　of　the　daughter　droplet　size　on　the　operation　conditions.　The　comparison　results　suggested　that　the　different　restriction　effects　of　microchannel　wall　on　daughter　droplets　led　to　the　different　breakup　mechanisms　in　different　constrictions.