An exemplary stereoscopic display device includes a backlight module, a display panel, and a polarizer panel. The backlight module provides a light source for the stereoscopic display device. The display panel is used for displaying a received image. The polarizer panel includes a plurality of polarizer elements. The states of the polarizer elements are switched by an ON/OFF operation for switching a polarization angle of the polarizer panel. The ON/OFF operation of the polarizer panel includes an enabled time interval and a disabled time interval. The polarizer elements provide two different polarization angles respectively in the enabled and disabled time intervals. The enabled time interval and the disabled time interval have different time lengths from each other. A stereoscopic image displaying method is also disclosed.