As an efficient heat and mass transfer technology, falling film evaporation has attracted more and more scholars' attention. In this paper, a falling film evaporation platform was established to discuss the mechanism of two-phase flow in narrow channels. The width of the channel varied from 0.5 to 2.0 mm. We focused on the relationship between heat transfer characteristics with flow patterns, channel width, and pressure loss. Three kinds of flow patterns were observed: bubble flow, restricted bubble flow, and dry area. In channels with the same width, the initial heat flux of subcooled evaporation increased linearly with the growth of flow rate. But at the same flow rate, the initial heat flux of subcooled evaporation decreased when the channel width became larger. The heat transfer coefficient on the wall was influenced by the bubble generation frequency and bubble separation diameter. Higher bubble generation frequency led to smaller bubble separation diameter. Through mathematical analysis, it was found that the heat transfer coefficient changed with the channel size. And a boundary was observed between the strengthening and weakening status. The comparison of theoretical and experimental values showed good agreement.