As the main flow type in the falling tube evaporator, the columnar flow has the advantages of stable shape, easy to control, moderate film thickness, high heat transfer coefficient, continuous liquid film disturbance and good heat transfer stability. In order to study the distribution of liquid film thickness and heat transfer coefficient outside horizontal tube in the stable column flow conditions, a 3-D model of falling film flow and evaporation in a wavelength range is established on a horizontal tube, and the CFD software is used for numerical simulation. Concluded as follow, the liquid film is spread along the axial and circumferential directions at the same time, and a ring liquid film peak is formed at the center of the wavelength. The film thickness of the peak is much larger than that of the non-peak region. The minimum film thickness appears near the circumferential angle of 120°, and the spray density has different effects on the peak and non-peak area. The distribution of heat transfer coefficient is affected by the film thickness and the fluctuation of the liquid film. The spray density has no obvious effect on the heat transfer coefficient.