In high rockfill dam, particle breakage is one of the main factors that lead to dam deformation. However, because of the large size of the rockfill particles, the breakage degree of the prototype rockfill materials is difficult to be measured directly through laboratory test. Therefore, the common practice is to reduce the size of the prototype gradation particle to less than 60 mm before the laboratory test can be carried out. However, due to the significant difference between the prototype and the test gradation, the parameters measured in the test are often quite different from the actual parameters of the prototype rockfill materials, thus affecting the in-depth study of the mechanical properties of the prototype rockfill materials. In this paper, a new method is proposed to describe the change of particle size distribution (PSD). Firstly, based on the theory of Weibull distribution of the particle strength and fractal crushing of the particle, the calculation of PSD change of the original rockfill materials is elaborated. Then, the relevant parameters are obtained by conducting the single-particle crushing test, and by comparing with the triaxial test, the rationality of parameter selection is verified. Moreover, the effects of the discrete degree of particle strength on the shape of the PSD is analysed. Finally, the relationship between the relative crushing parameters and stress state of the rockfill materials during loading is discussed.