There is provided an image system including at least one image sensor, a light source and a processing unit. The at least one image sensor sequentially acquires at least two images within each of the brightness variation intervals of an ambient light source. The light source lights once within each of the brightness variation intervals and the lighting of the light source is synchronized to one of the images acquired by the at least one image sensor. The processing unit calculates an image difference between the image synchronizing to the lighting of the light source and the image not synchronizing to the lighting of the light source thereby eliminating interference from the ambient light source. There is further provided a denoising method for an image system.