The　coalescence　of　two　droplets　with　unequal　lengths　through　a　head-to-rear　collision　in　the　microchannel　with　a　double　T-junction　is　an　important　passive　method　to　construct　droplet　reactors.　However,　the　accurate　prediction　of　the　coalescence　position　is　still　not　achievable.　This　work　aimed　to　explore　the　coalescence　position　by　analyzing　the　droplet　dynamics.　Firstly,　the　velocity　variation　of　the　paired　droplets　moving　in　a　straight　microchannel　was　analyzed　to　divide　the　whole　process　into　several　stages.　Then,　based　on　the　experimental　investigations,　mathematical　models　were　developed　to　describe　the　distances　of　the　droplet-pairs　moving　in　each　stage　to　predict　the　coalescence　position　in　the　straight　microchannel.　In　addition,　the　coalescence　position,　as　well　as　the　coalescence　mechanism,　was　analyzed　for　the　paired　droplets　in　an　expansion　microchannel　to　explore　the　more　possibilities　of　intensifying　the　construction　of　droplet　reactors　in　microchannels.