A display device for a user to observe a stereo image through a pair of polarization glasses includes a projector, a switchable polarizer, and a sensor. The projector sequentially displays a first image, a first middle image, a second image, and a second middle image on a projection screen. The switchable polarizer is located between the projector and the projection screen and is switchable between a first mode and a second mode, so that each of the first and second images has a first or second polarization after passing through the switchable polarizer in the first or second mode and is then projected onto the projection screen. The sensor is coupled to the switchable polarizer and senses the images on the projection screen. When the sensor senses the first middle image or the second middle image, the switchable polarizer switches between the first mode and the second mode.