Al-Cu-Mn-Mg alloys are widely used in aerospace, rail transportation, and other fields because of their excellent comprehensive properties. However, this alloy is prone to hot cracking defects during the casting process, which severely affects the quality and reliability of castings. Therefore, improving the grain refinement effect of an alloy is highly important for enhancing its casting performance. The Al-Ti-B refiner is the most commonly used grain refiner in aluminium alloy casting, but the effects of Al-Ti-B refiners with different Ti-B mass ratios on grain refinement and hot cracking mechanisms have not yet been systematically studied. The influence of Al-Ti-B refiners with different Ti/B mass ratios on the microstructure and hot cracking susceptibility (HCS) of cast Al-Cu-Mn-Mg alloys was investigated via X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), and constrained rod casting (CRC) metal molds restrained by hot cracking rod molds. The experimental results reveal that the contents of the Al3 Ti and TiB2 phases in the four Al-Ti-B refiners (Al-3Ti-1B, Al-4Ti-1B, Al-5Ti-1B, and Al-5Ti-2B) significantly differ. At a constant B content of 0.3 wt.%, the average grain sizes of the Al-5Cu-0.7Mn-0.55Mg alloy refined by these four refiners are 162.07, 113.73, 107.81 and 88.00 μm, respectively. The corresponding HRC indices are 112, 136, 96 and 24. Microstructural mechanism  analysis reveals that the content of the TiB2 phase significantly influences both the grain refinement efficiency and hot cracking susceptibility. Notably, the Al-5Ti-2B refiner demonstrates superior performance by simultaneously achieving effective grain refinement and substantially reducing the hot cracking tendency of the alloy.