A power-saving method adapted in a multi-mode display device is provided. The power-saving method comprises the steps as follows. A brightness of an ambient light is detected. When the brightness of the ambient light lies within a transmissive range, the multi-mode display device is operated in a transmissive mode and enables the backlight module of the multi-mode display. When the brightness of the ambient light lies within a transflective range, the multi-mode display device is operated in a transflective mode and the brightness of the backlight module is dynamically adjusted according to a compensation method. When the brightness of the ambient light lies within a reflective range, the multi-mode display device is operated in a reflective mode and turns off the backlight module.