A thin metal film is introduced to split two-way polarized image beams, thereby accomplishing a stereoscopic three-dimensional display. Linearly polarized beams emitted by two uniform liquid crystal displays (LCDs) are reflected and transmitted by ultrathin aluminum film separately. Since optical constant of ultrathin metal film depends on film thickness, a piecewise linear function is used to obtain the relationship between the volume fraction of aluminum and the thickness of the film, and then the optical constant can be estimated by the Sheng Model. Following this procedure it is proved that both the reflected and the transmitted beams become elliptically polarized light beams and their principal axes are approximately vertical. Thus two orthogonal polarizers can be used to separate them and then make the observer have a perception of depth in LCD image. In order to keep the quantities of light entering the observers two eyes balanced, the thickness of the aluminum film is optimized. Experimental results are in agreement with the theoretical analyses, so the correctness of the method is verified.