In X-ray digital radiography system based on the image intensifiers, there are many kinds of noise which originate from different imaging elements, such as CCD camera, scintillator photoconvertor, X ray scatter, control circuit, and so on. These kinds of noise have a serious influence on the imaging quality. It maybe influence the result of identifying defects from the image and the spatial resolution of X-ray digital radiography system. In the paper, in order to depress the noise of system, the origin and the characteristic of the noise in X-ray digital radiographic system are studied, and a method and examples are given to depress the noise of X-ray digital radiograph on the principle of M channels' wavelet transforms, multiresolution wavelet analysis and control of coefficient in wavelet. It is shown that the results of noise depression are effective, the spatial resolution of image is improved, the edges and details of image are enhanced. It is also expressed that the method to depress the noise of X-ray digital radiography system by softer is feasible and economical.