A holographic data storage simulator for the combined multiplexing method which consists of spatial shift, peristrophic,
and focus-shift multiplexing has been developed. In the combined multiplexing method, wedge prisms were used to
realize the focus-shift multiplexing, so we gave linear phase modulations on the reference beams to represent the effect
of wedge prisms in the simulation and successfully got a good agreement between the simulated and experimental results
on the diffraction efficiency characteristics. The diffraction efficiency decreases rapidly as the focal position of the
reference beams are shifted, which indicates that it is possible to enlarge the capacity of the holographic data storage by
the focus-shift multiplexing.