Computer simulation of the variable output beams produced by a single double-layer rectangular phase grating is presented. With a single element, 2, 3, 4 or 5 output beams with 72.93%, 82.48%, 71.20% or 83.04% of total diffraction efficiencies respectively can be obtained by tuning the read out angle of an incident beam. Methods for improving the optical properties of this element are discussed, and the potential applications in optical memory and optical communication are suggested.