A compressing method is applicable to compress of an image at a fixed compression ratio, in which the image has a raw data. The compression method includes the steps of receiving the raw data with a predetermined length as a compression unit; compressing the compression unit into a compressed bit stream, wherein a total length of the compressed bit stream being not larger than a target bit stream length; when the total length of the compressed bit stream is smaller than the target bit stream length, appending a dummy code to the compressed bit stream, so that a bit stream length of the compressed bit stream with the dummy code is equal to the target bit stream length; and repeating steps until the raw data is compressed into an image bit stream.