Digital micro-mirror devices (DMDs) as spatial light modulators, have the advantages of high modulation speed and accuracy and can perform amplitude or phase modulation for illumination light, which are widely used in phase imaging systems. In this paper, a DMD is used for the random coding modulation of illumination light amplitude. We simulate the influences of the smallest modulation unit and switch ratio on phase imaging when random coding patterns are loaded on the DMD. Then we use a difference mapping iterative algorithm for light field reconstruction. Furthermore, it is experimentally verified that based on the parameters obtained via numerical simulation, we can retrieve amplitude and phase with a higher iteration speed and a fewer measurement times. In addition, for an object with the field of view of 4.3 mm×4.3 mm, the reconstruction resolution reaches 6.9 μm.