Horizontal light field three-dimensional (3D) display is an important 3D display method, it can achieve fairly good 3D display effect when remarkably compressing the mass information of light rays that need to be processed. As there is no change in vertical light field, its expressive force of display definition and the ability of monocular focusing for horizontal light field are both different from the ideal light field. The point spread function (PSF) of the horizontal light field is analyzed based on the scanning light field display. Based on the auto-focusing principle of human eye and camera, the horizontal light field that is reconstructed under different display parameters is focusing simulated, which shows that the effect of horizontal scattering angle of light and the image number per screen spinning circle of the projection on the monocular focusing ability in horizontal light field is obtained. The conclusion is verified by using a scanning horizontal light field display prototype. And the results show that the maximum focusing height of horizontal light field declines with the increasing of horizontal diffusing angle of light ray, and it goes up with the increasing of the image number per screen spinning circle of the projection. The proposed analytical method and the conclusion can provide the design consideration for the horizontal light field display system of monocular focusing.