The invention provides an ambient light sensing device which receives at least one visible light image sensed by an image sensor. The ambient light sensing device includes an image sampling unit and an analyzing unit. The image sampling unit divides the visible light image into plural image blocks, extracts at least one sample data in each image block, and generates a comparison data according to a difference between the sample data extracted at different time points. The analyzing unit analyzes the comparison data and generates an output analysis signal accordingly.