Forward osmosis (FO) membranes with high water permeation flux are required for FO processes to overcome reverse osmosis energy efficiency and translate the FO process into practical applications. However, in the case of such membranes, convective solute transport has a profound effect on solute permeation flux. The existing solution-diffusion model ignores the convection term and the solute permeation flux cannot be accurately analyzed. To solve this problem, the FO membrane permeation was theoretically modeled via non-equilibrium thermodynamics to introduce the convection effect and by coupling with both the internal and external concentration polarizations. First, the FO desalination process (feed solution was sea water (3.3 wt% NaCl aq.)) was investigated by the model. By slightly decreasing the reflection coefficient, which had minimal influence on water permeation flux, both the convective salt transport and NaCl permeation flux increased significantly. Second, a simplified model system (the feed solution was pure water and the draw solution was NaCl aq.) for the wastewater treatment FO process was analyzed. In this system, water permeated in a direction opposite to that of NaCl, and relevant complex behavior of salt permeation was observed.