1. An x-ray imaging system for performing an x-ray scattering imaging of an examined object, comprising: an x-ray source, a first and a second absorbing gratings, an x-ray detector, wherein the x-ray source, the first and second absorbing gratings are orderly located on a propagation direction of the x-rays; a data processor unit for processing data information; a reconstruction unit for reconstructing an image of the examined object; and a controller for controlling operations of the x-ray source, the first and second absorbing gratings, the x-ray detector and the data processor unit, so as to carry out the following processes: the x-ray source emits x-rays to an examined object; one of the two absorbing gratings performs phase stepping motions within at least one of its cycles; in each phase stepping motion, the detector receives the x-ray and converts it into an electric signal; wherein through the phase stepping motion of at least one cycle, the x-ray intensity at each pixel point on the detector is represented as an intensity curve; the intensity curve at each pixel point on the detector is compared to an intensity curve in the absence of the examined object, to acquire a variation in the contrast ratio of the intensity curves; a second moment of scattering angle distribution for each pixel on the detector is calculated from the intensity curve variation; and pixel values of the examined object are obtained from the second moment of scattering angle distribution, whereby an image of the examined object is reconstructed from the pixel values of the examined object obtained from the second moment of scattering angle distribution.