Digital simulation of optical experiment has been more widespread in resent years. In this paper, diffraction angular spectrum theory is used to calculate the intensity distribution of the diffractive optical elements. This theory can be expressed by transform function, and also can be expressed by point spread function. However, two expressions vary inMATLAB. We could divide them into near field simulation method and far field simulation method. A consequence of experiment is given and discussed. It shows that they can calculate distribution of illumination in different diffractive distances accurately. These methods are important for diffraction simulations.