A weighted prediction image encoding method includes: calculating a first feature value of a first frame, a second feature value of a second frame and a third feature value of a third frame; calculating a first offset value according to the first feature value and the second feature value, and calculating a second offset value according to the second feature value and the third feature value; and when both of the first offset value and the second offset value are greater than or less than a predetermined value, encoding the third frame by utilizing weighted prediction operation, wherein the first frame, the second frame and the third frame are multiple frames in chronological order.