This work presents a numerical study on the pressure retarded osmosis (PRO) using computational fluid dynamics (CFD) modelling. First, a numerical solver based on CFD to describe the mass transfer across the membrane and the hydrodynamics in the flow channels is developed and validated with the previous publications. Several key steps of the implementation in OpenFOAM are also discussed. Using the validated CFD solver, different operating parameters are investigated by simulations. According to the obtained concentration and flow fields with different applied pressures, a lower hydraulic pressure causes a more significant external concentration polarization (ECP) effect at the draw side. In addition, effects of the cross-flow velocities are also studied with respect to the ECP and the overall performance of the PRO. Furthermore, two models of the PRO, CFD model and semi-analytic model, are compared. The results indicated the good performance of the semi-analytic model to approximate the overall performance of the PRO with a relatively lower permeable membrane. However, with the improvement on the membrane properties, large deviations of the averaged membrane power density are found between the two models.