Abstract: A flat model is used to further study the scatter law in the X-ray radiography. The results are obtained with the analytic method and the MC simulations. One shows that in the radiography the Compton scatter is the dominant mode of interactions between the photon and the material and the first Compton scatter is more than 75 percent of the total scatter when the optical path of the flat is less than 2.0. And the analytic model is established effectively with the first Compton scatter approximation to the total. Another is that the same conclusion, which is the optical path of the flat is between 1.0 and 2.0 when the maximum scatter happens, is gained by analytic method, MC method with first Compton scatter and MC method with total scatter, respectively.