Desalination by freezing is of great prospect due to its low energy consumption, little pollution, freedom of corrosion and ease of scaling. A novel freezing desalination method driven by humidity difference between air flow and liquid surface is proposed and verified in this paper. In order to reveal its mechanism, an unsteady-state thermodynamics model was established to simulate icing movement on liquid surface according to heat and mass balance. Effects of humidity difference, airflow velocity and mass diffusion coefficient on icing development were analyzed through theory analysis and experiments. It was found that the larger humidity difference and larger airflow velocity had evident effects on the liquid evaporation-freezing. Meanwhile, fast mass diffusion between mediums was also beneficial to this freezing process.