Stereo depth is the most important factor for the 3D experience when viewing an autostereoscopic display. In this paper, we investigate the influence of viewing distance and viewing angle on stereo depth. First, we build the ideal stereo depth model based on the physiological limitation. Second, we establish a wave aberration model based on diffraction theory. The simulation and experimental results agree with the theoretical analyses. The model is of significant importance for giving a guidance on display system designing.