A stereoscopic image displaying method and a stereoscopic display device thereof is provided. The stereoscopic image displaying method includes a data providing step, a polarization adjusting step, and a backlight controlling step. The stereoscopic display device includes a display panel, a light modulation device, and a backlight module which are respectively configured to perform the data providing step, the polarization adjusting step, and the backlight controlling step. In the backlight controlling step, the time of turn-on and turn-off is adjusted. Alternatively, in the data providing step, black-image data is added.