The mobile video playback involves many subsystems of the devices such as computing, rendering and displaying subsystems. Among all subsystems, the displaying subsystem accounts for at least 38% of all consumed power, and it can be up to 68% with the maximum backlight brightness. What is more, lots of people watch videos via mobile devices in various situations, where the ambient luminance condition is different. Therefore, how to save mobile energy and improve the Quality of Experience (QoE) in different situations become significant problems. In this paper, we try to maximally enhance the battery power performance under various ambient luminance conditions through backlight magnitude adjusting, while without negatively impacting users' QoE. In particular, we conduct a series of subject quality assessment experiments to uncover the quantitative relationship among QoE, ambient luminance, video content luminance and backlight level. We first study whether the continuous playback of backlight-scaled shots using the proposed scaling magnitude would cause flicker effect or not. Then motivated by the findings of these subject studies, we implement a Dynamic Backlight Scaling (DBS) strategy. The experiment results demonstrate that the DBS strategy can save more than 40% power at most and can also save 10% power even at a very high ambient luminance.