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; 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 image of the examined object are obtained from the second moment of scattering angle distribution, whereby the image of the examined object is reconstructed.