In the glasses-free 3D stereoscopic display technology based on the cylindrical lens grating, there is no clear geometric model of the viewing area. Firstly, a geometric model of the observation area of the glasses-free 3D display system was established, and the spectroscopic effect of the lenticular grating plate was studied. The relationships between the lenticular parameters, the observation distance and the observation angle were analyzed. Then the geometrical model was used to analyze the formed observation area, and the expression of the viewing range and the relationships between the lens parameters, the display screen parameters and the viewing range were obtained. Finally, the quality factor concept was combined with the subjective evaluation method to evaluate the image quality in the viewing area of two glasses-free 3D display devices. The image quality in the observation area was evaluated. The experimental results showed that the best viewing area of the glasses-free 3D display could be calculated according to the established geometric model. This model provided a reference for the optimal design and specific implementation of free-stereoscopic displays.