The long-term safety of rockfill dams is critically dependent on the quality of the filling material. Therefore, it is essential to predict the blasting grading distribution scientifically and accurately. This paper proposes an engineering-scale numerical simulation method to predict the grading distribution of hydraulic-grade rockfill material and verify its effectiveness through major hydropower engineering applications. Firstly, based on the mechanical mechanism of rock fragmentation and relevant experimental data, the critical blasting damage threshold for rock detachment is determined. Then, the threshold is used to identify the boundaries of blasted rock blocks, enabling the quantitative derivation of grading curves. The method is applied to simulate the blasting mining of fill materials at the Lianghekou Hydropower Station. A comparison with field screening data demonstrates that the method effectively simulates the spatial distribution of blast-induced block sizes. Furthermore, numerical simulations investigate the influence of hole arrangements and delay times on fragmentation. The results indicate a significant difference in the grading distribution characteristics between rectangular and staggered hole arrangements, with the rectangular arrangement conducive to producing a continuously non-uniform gradation. Delay time also markedly impacts fragmentation, primarily affecting the distribution of medium and large-sized blocks. The mean fragment size initially decreases and then gradually increases with longer delay times.