Abstract: The Kelvin Helmholtz instability of evaporating falling film flow in vertical tube is studied by means of energy analysis. Based on the rules that the interfacial capillary waves come from the balance of works done by inertial force, surface tension on phase change interface, and also capillary force on tube wall, the stability behaviors of falling film with different Reynolds numbers and perturbation wavelength are explored in detail. The analysis indicates that the main reason of film breakup is that, the stability effect of capillary adsorbability on tube wall is weakened by increasing tube wall heat flux.