The invention provides a new judgment method for light gradient of a picture for a film source with illumination variation. The method comprises the steps of calculating probability distribution of luminance information of the current picture and two reference pictures, and then detecting the picture with the light gradient by using variation characteristics of the probability distribution of the three pictures. As the method uses the reference pictures to assist in judgment, the method can be rapidly combined with a video encoding and decoding technique; in addition, the method only uses the variation of the probability distribution of the three pictures for judgment, and the method has the characteristics that the calculation load is small and the implementation is convenient.