Theoretical studies have been made to analyze combined refraction diffraction of the wind waves propagating on a large scale current in water of varying depth. The governing equation for monochromatic waves was derived through splitting a mild slope equation into two equations. A numerical model is developed using finite difference scheme which is computationally very efficient for modelling large area. Numerical examples concerning the interactions between waves and rip currents over a gentle slope are presented, in which the current effects on the wave diffraction in the caustic region are closely examined.