A flicker detection method of an image sensing device is disclosed. The image sensing device includes a sensor and a processor. The method comprises steps of: sequentially detecting multiple frames according to a frame rate, wherein each of the multiple frames includes a light signal; generating a light intensity information based on the light signals; determining a sampling window according to a predetermined detection frequency and the frame rate; dividing the light intensity information into multiple light intensity groups according to the sampling window; distinguishing a feature of each light intensity group, and recording an index position of each feature of the light intensity groups; calculating, individually, a difference between the index positions of the adjacent light intensity groups; and determining whether the differences are patterned, in order to determine whether the light intensity information corresponds to a flicker.