There are two sampling models of digital hologram in digital holographic research. The complex mathematical operation is involved in these two models, thereby leading to different results frequently and inconvenient theoretical analysis and practical application. Based on the analysis of physical processes in recording digital hologram and the study on geometry structure of CCD detector, the two sampling models are simplified into the same mathematical expression in this paper. Through studying the simplified model with cosine series expansion of digital hologram and sampling theorem, it is shown that the model has clear physical meanings and can be used for application study conveniently. Using the simplified model, we study digital holographic recording system with optical system and exact wavefront reconstruction expression verified in experiment.