Abstract: The influence of rock joints on the formation of rock blast fragmentation has always been a hot issue in blasting engineering, which is one of the most important factors affecting the construction efficiency and long-term operation safety. Based on the exploiting tests on the graded materials of Yulong Kashi Water Conservancy Project, the influence of rock joints on the formation of rock blast fragmentation was studied. The test results show that rock particle size below 5 mm is mainly formed by blasting load and dynamic compressive strength, but rarely by the rock joints. However, the influence of the rock joints on rock particle size above 5 mm is significant, which is reflected in the partition characteristics for the formation of rock blast fragmentation. On the other hand, the numerical simulation of rock blast fragmentation is carried out by LS-DYNA secondary development technology, and the mechanical mechanism of the rock joints impacting on rock blast fragmentation is analyzed. Results show that the rock joints greatly affect the stress wave propagation in the fracture zone but produce a small effect on the formation of small rock particle size, which is verified by the test results. Based on the dual influence of rock blast fragmentation and rock joints, a partition prediction model for the rock blast fragmentation is proposed. The rock blast fragmentation for the many test samples is predicted by the proposed model and the existing models. By comparing the predicted results, it is found that the proposed model in this paper has a higher accuracy in predicting rock blast fragmentation.