The physical and mathematical models of a capillary tube at constant heat flux are established. The two-phase flow mechanism consists of the core vapor flow and the liquid film flow, which are driven by the gradients of capillary pressure and disjoining pressure. And there exists the vapor-liquid interaction of shear stress at the interface, which is due to both the velocity difference of vapor and liquid, and the momentum transfer of evaporation. The heat transfer mechanism is composed of the liquid film conduction and the evaporation at the vapor-liquid interface. In the presented models, the evaporating interfacial region is not subjectively divided into several sub-regions, the thickness of the liquid film is solved by the governing differential equations. The effects of the applied heat flux and the radius of capillary on the profile of liquid film are calculated and compared respectively. With the increase of heat flux and capillary radius respectively, the length of evaporating interfacial region decreases, the former factor has more significant influence on both the meniscus position in the capillary tube and the thickness of liquid film.