Abstract: This study investigates the coupled breakage and transport mechanism of rock chips in the crushing box of a slurry shield machine based on the Qingdao Jiaozhou Bay Second Undersea Tunnel. A three-dimensional full-scale CFD-DEM model incorporating the periodic motion of the jaw crusher, particle breakage, and slurry flushing was established, with the Tavares UFRJ model used to describe particle fracture. The results show that rock-chip breakage is governed by a combined compression-impact mechanism: compression-dominated breakage mainly occurs between the crushing arms, whereas impact breakage is concentrated near the lower part of the crusher. The periodic swing of the crusher induces cyclic redistribution of slurry dynamic pressure and promotes particle transport toward the grating and discharge pipe. Increasing the rock-chip mass flow rate enhances both breakage and discharge, but excessive feeding leads to particle accumulation and a higher blockage risk; therefore, a range of 10.5-13.1 kg/s is recommended. Increasing the slurry flushing flow rate improves particle discharge but reduces residence time and breakage completeness. Accordingly, flushing flow rates of 150-180 m3/h and 180-200 m3/h are recommended for highly fractured and relatively intact rock masses, respectively. The findings clarify the breakage-transport coupling mechanism and provide guidance for parameter optimization and blockage prevention in slurry shield secondary crushing systems.