1. A method for estimating noise according to a multiresolution model, wherein the method is applied to an imaging device and comprises steps of: using an imaging sensor of the imaging device to capture a series of images of a specific scene under different imaging conditions; processing each of the images with a multiresolution transformation, so as to obtain a series of sub-images corresponding to different frequency layers; processing a series of said sub-images of the series of images that are in a same frequency layer, in order to generate an averaged image, which is defined as an actually noiseless sub-image sample in that particular frequency layer; determining a difference between each of the sub-images in the same frequency layer and the averaged image corresponding to that particular frequency layer, and calculating said differences and the averaged image to obtain a noise level function of the imaging sensor in each of the different frequency layers under the different imaging conditions; and defining the noise level functions of the imaging sensor in the different frequency layers under the different imaging conditions as noise samples for building up an a priori model database.