In order to investigate the improvement of the rock breaking ability of a disc cutter under hard rock conditions, the use of a water jet-assisted cutter to break rock was explored as a promising new rock breaking technology. The impact of slit spacing on the effectiveness of slit-assisted cutter rock breaking was verified through rock breaking tests in this study. It was demonstrated that, under specific conditions of rock material, penetration depth, and slit depth, there exists a threshold spacing for slit-assisted cutter rock breaking. Only when the slit spacing is less than the threshold spacing, can penetration crushing occur on the slit side. Additionally, the rock shear damage theory was applied to analyze and propose a linearized threshold spacing prediction model, which indicates that the threshold spacing is linearly correlated with the difference between the slit depth (L) and the penetration depth (H), denoted as (L - H).