Abstract: In order to solve the problems of difficulty in tunneling, serious tool loss and low efficiency of breaking ultra-high strength rock encountered in the construction process of traditional hob, high-pressure water jet has become a new method to improve the efficiency of rock breaking beacuse of its good crush effect on hard rock. The arrangement of water jet and cutter would directly affect the force of cutter and the specific energy of rock breaking. Aiming at the reasonable determination of the optimal rock breaking impact angle of water jet assisted hob, a discrete element model of equivalent crystalline rock material based on Thiessen polygon algorithm was built. PFC（Particle Flow Code） was used to numerically analyze the variation laws of crack number, rock stress and specific energy of rock breaking under different water jet impact angles.These can further obtained water jet impact angle on rock breaking efficiency of disc cutter. The results show that: The bottom of the notch is the stress concentration area in the process of rock breaking by the hob, the existence of pre-cut slit affects the crack to preferentially expand to its bottom. The increase of pre-cut slit length can reduce the hob load, but the effect is gradually weakened with increase length of the pre-cut slit, and the normal load of the rock sample with pre-cut slit is reduced by 6.11%−17.60% compared with that without slit. The increase of pre-cut slit length can reduce the hob load, but the effect is gradually weakened with increase length of the pre-cut slit. There is a threshold value of 20 mm for the influence of the pre-cut slit length on the specific energy of rock breaking, beyond this threshold value, the influence ability of pre-cut slit length on the failure of the rock sample is weakened. Change of the impact angle of the water jet can significantly change the rock breaking effect of the hob. The specific energy of rock breaking develops in a “V” shape curve with the increase of impact angle of the water jet. The best rock breaking effect is obtained with the assistance of water jet impact angle of 75°. The reasonable selection of water jet impact angle can contribute to improve the TBM tool life. Results can provide theoretical reference for the arrangement of water jet system and nozzle angle in TBM cutter head structure under hard rock geological conditions.