During the simulation of material mechanical properties of granular materials using the discrete element method(DEM), in order to guarantee the continuity of the generated grading curve for the numerical specimen and the conformity of the generated grading curve with the desired grading curve, a new method based on the probability density function (PDF) was proposed to generate a numerical specimen of the particle flow DEM. A parameter to describe the mismatch of grading was defined to quantify the deviation between the generated grading curve and the original grading curve. A comparison of the proposed method and the existing method of specimen generation was conducted and the influence of the particle number on the generated grading curve was investigated. The results show that the proposed method can generate specimens of an arbitrary grading curve and the generated grading curve has a high degree of continuity and smoothness. Moreover, as the number of generated particles increases, the generated grading curve based on the PDF converges to the original grading curve.