The coalescence between liquid flow and droplets is usually encountered in liquid filling or some new microdevices; a deep understanding of its underlying mechanism is required. In this work, therefore, the dynamic behaviors of this atypical phenomenon in microchannels were visually investigated with droplets generated by photothermally induced evaporation and condensation. The effects of droplet quantity and position, inlet pressure, and microchannel size were also explored. Experimental results showed that the coalescence accelerated the liquid movement as a result of lowered pressure at the interface. Parametric studies indicated that large droplet quantity and small distance between the inlet and droplets yielded a large velocity increment ratio as a consequence of lowered liquid pressure at the interface and increased pressure gradient, respectively. A high peak velocity increment but with low velocity increment ratio was obtained at high inlet pressure because of high liquid flow velocity. Moreover, the microchannel with a small size yielded a strong acceleration rate of the liquid movement because of more obvious capillary action in the small channel. Results obtained will be helpful for the design and operation of those kinds of microdevices that may face this atypical phenomenon.