The　mechanism　of　microdroplet　coalescence　is　a　fundamental　issue　for　droplet-based　microfluidics.　We　developed　an　asymmetric　expansion　(a　rectangular　groove)　along　one　side　of　a　microchannel　to　achieve　multiple-microdroplet　trapping,　collision,　and　coalescence.　Compared　with　reported　symmetric　expansions,　this　asymmetric　groove　could　easily　trap　microdroplets　and　control　two　or　three　microdroplet　coalescences　precisely　without　a　requirement　for　temporal　and　spatial　synchronization.　To　reveal　the　mechanisms　of　multiple-droplet　coalescences　in　a　groove,　we　observed　five　different　coalescence　patterns　under　different　flow　conditions.　Moreover,　we　characterized　the　flow　behavior　quantitatively　by　simulating　the　velocity　vector　fields　in　both　the　microdroplets　and　continuous　phase,　finding　good　agreement　with　experiments.　Finally,　a　map　of　coalescence　forms　with　different　capillary　numbers　()　and　flow　ratios　()　was　obtained.　The　results　could　provide　a　useful　guidance　for　the　design　and　application　of　droplet-based　microfluidic　devices.