On the basis of lattice wave theory, the several side-band profiles of X-Ray diffraction of spinodal decomposition for Cu-4wt%Ti alloy are simulated by computer through supposing the rectangular distribution of solute atoms in the alloy, and the profiles are extremely similar to experimental results. Moreover, in the light of the principle of superposition, the composition of the above-mentioned profiles which are perpendicular to each other in two directions is carried out and the simulated diffraction patterns obtained are extremely good consistent with TEM diffraction pattern.