There is provided an optical sensor including a photodiode, a wave converter, a pixel array and a processor. The photodiode detects ambient light flicker to generate sine waves. The wave converter converts the sine waves to square waves. The processor uses a sampling frequency to count the square waves, and identifies whether the ambient light flicker is well detected according to a counting value of each square wave and a counting value variation of multiple square waves within a count period to accordingly determine whether to adjust an acquiring phase of the image frame.