A method for progressive JPEG image decoding and a method for controlling a decoder are disclosed. In the image decoding method, an amount of macroblocks that can be stored during the process of decoding is determined according to the size of the memory, and then, according to the amount of macroblocks, a scan layer is divided into a plurality of decoding areas. In each turn of decoding, data of the same decoding area in all the scan layers are decoded. In the next turn, a decoding area next to the previous one is selected for decoding. Therefore, the present invention can decode the image correctly with a limited memory resource, without affecting the display quality of the image. The image decoding method also can be applied to the control of a decoder, so as to reduce the occupied memory capacity.