The amendatory Bragg formula may be used for calculating the periodic thickness of multlayers deposited on the plane substrate by small X-ray diffraction spectrum. However, the diffraction peak angle of multilayer deposited on the curved substrate will be different from that of multilayer fabricated on the plane substrates though they have the same periodic thickness. Thus, the calculated periodic thickness of the above multilayer with the Bragg formula is not well agreement with the real value of the multilayer. This paper gives an analysis of phenomenon of diffraction peak excursion, and gives a revised formula derived from the amendatory Bragg formula for accurately calculating the periodic thickness of multilayer deposited on any substrates. The experimental results show that the periodic thickness calculated with the formula is consistent with the practical periodic thickness.