A two-dimensional, sharp-interface two-phase laminar flow numerical model for evaporation from a saltwater falling liquid film with a co-current humid air flow on the interior of a parallel plate vertical channel is presented. The elliptic governing equations in both phases are solved simultaneously in an implicit, fully coupled approach. The liquid-gas interface is tracked precisely on an automatically adjusted non-orthogonal structured mesh. The basis of the evaporation model is validated for pure water falling film evaporation. A parametric study of evaporation from a saltwater falling film is presented. The study varies the inlet salinity of the film, the inlet liquid Reynolds number, the wall heat flux, and the air relative humidity. The effects of these variations on quantities such as the evaporation rate, the axial velocity, the salinity in the film, and the temperature are presented. The results demonstrate the capablity of the new model to predict the details of falling film thermal desalination.