Taking deionized water as working fluid, an experimental study was conducted to investigate the heat transfer characteristics of falling film evaporation outside horizontal aluminum tube with inner diameters of 10 mm and 14 mm. The effect of spray density, heat transfer temperature, evaporation temperature, tube diameter, outlet vapor quality and degree of undercooling on overall heat transfer coefficient outside the tube was analyzed. The results show that the overall heat transfer coefficient increases with increasing evaporation temperature and tube diameter while decreases with increasing heat transfer temperature difference and outlet vapor quality. On other hand, the overall heat transfer coefficient increases with increasing spray density firstly, then decreases slightly. The effect of outlet vapor quality can be neglect.