Abstract: The transporting process of Xray's photons in the spheresymmetric object is simulated with the MonteCarlo method. Ｔhe simulated results of spectra of the output photons on the recording plane are presented,including flux spectra and exposure spectra of the direct p hotons and the scattering ones, in cases of single object and full system. It is shown that the spectrum on the border of object is in analogy to the bremsstrahlung spectrum of the target. The spectra for the region with longer optical length are nearly the same, and so it is with the spectrumaveraged photon energy for the region. This energy is in the regime wh ere the attenuation coefficient of the high Z element is the smallest. It is also shown that, because the quantity loss of photons varies with photon energy, th