In this paper, according to the H-D curve of silver halide plate, we systematically analyse and explain the various phenomena which are produced by the zeroth and other orders diffraction images. The reversal conditions are given and some important conclusions are obtained. In our experiment, the density distributions of encoded plates are measured by using microdensitometer and corresponding distributions of diffraction intensity are recorded. The experimental results are consistent with theoretical analysis. This paper will provide foundations for making use of the characteristics of grating encoded plates.