Abstract: A gradation equation is used to describe the four grading scale methods in the Chinese standard, which are exclusion method, similar grading method, equivalent substitution method and hybrid method, and a new and unified equation is put forward to calculate the gradation after reducing the scale. The relationship between the parameters (d_max, b, m and C) of the new equation and those of the old equation (d_0max, b₀ and m₀) is studied and revealed. The results show that the new parameter m of the four methods is m=m₀, and parameter C of the exclusion method and the similar grading method is C=0, and parameter b of the equivalent substitution method and the hybrid method is, and the parameter C of the equivalent substitution method is decided by and P₅. This method is simpler than the methods in the Chinese standard; and especially for the hybrid method, the fine particle content P₅ can be defined firstly. To some extent, the unified equation makes it possible to study the reduced scale effects quantitatively and systematically. At the same time, the reduced scale calculation of soil gradation for rockfill of Nuozhadu Dam is demonstrated according to this relationship.