Recently,　with　the　development　of　lab-on-a-chip　researches,　the　use　of　microfluidics　to　precisely　control　the　droplet　behaviors　in　microchannel　has　received　more　and　more　research　attention.　This　article　firstly　introduces　the　basic　theory　and　mechanism　of　the　droplets　coalescence.　Then,　it　carefully　reviews　the　passive　methods　of　droplets　coalescence　used　in　current　researches　by　changing　microchannel　geometry　and　adding　surfactants.　The　active　methods　to　trigger　droplet　fusion　such　as　applied　electric　field,　magnetic　fields,　temperature　gradients,　surface　acoustic　wave　and　focused　lasers　are　also　introduced　briefly.　We　also　introduce　the　fluid　dynamics　of　the　droplets　coalescence　and　forecast　its　progress　at　last　to　provide　useful　guidance　for　microfluidic　devices　design　and　wide　use　of　the　drop-based　microfluidics.