Calculation of reflectivities is foundation of computer simulation of X-ray rocking curve. The effect of HgCdTe composition ununiformity on X-ray reflectivities is studied. The results show that X-ray reflectivity rocking curves of HgCdTe materials are affected by transverse composition ununiformity. As composition ununiformity increases, peak value and full width at half maximum (FWHM) of X-ray reflectivity decrease and increase respectively in the exponential function of mean square deviation of composition ununiformity, but a integral reflectivities keep constant. The X-ray reflectivities of HgCdTe materials with a longitudinal linear composition gradient are calculated by using multilayer model. It is found that reflectivity peak values decrease and FWHM of the reflectivities increases as the increase of composition gradients. Besides of these, longitudinal composition gradients will induce some interference peaks at single side of the reflectivity curves. The interference wavelengths increase with the increase of composition gradient. And variation of interference wavelengths with interference period number also increases.