An image processing method applied to an image processing device is capable of implementing bitstream stitching technique after interrupting image processing process. The image processing method includes steps of processing the i-th slice of N slices in an image to generate a plurality of first processed data; storing the first processed data in a memory unit; once an interrupting request is generated according to a requested process, storing stitching information associated with the last first processed data after processing the i-th slice; stopping processing the image and executing the requested process according to the interrupting request; continuing to process the (i+1)-th slice of the N slices to generate a plurality of second processed data after the requested process is finished; and storing the second processed data after the last first processed data in the memory unit according to the stitching information.