The solution-steam-air multiphase flow model for heat and mass transfer in vertical tube failing film under gas-liquid cross flow condition is set up. Numerical simulation of heat and mass transfer process in single vertical tube falling film is performed. The distribution of falling film outside vertical tube and the rule of its heat and mass transfer at different air ve- locities are discussed. With the increase of horizontal air velocity, the rupture takes place at the windward side of the falling film outside the vertical tube, and the rupture point moves up rapidly with the increase of ai...