Enzyme-induced carbonate precipitation (EICP) is an emerging environmentally friendly soil stabilization technology. The mechanical properties of bio-cemented sand are highly dependent on particle size. However, the influence of particle size on the solidification effect and the underlying mechanisms remain unclear. To investigate the effect of particle size on EICP solidification, this study conducted a series of EICP sand column tests using sands with different particle sizes (coarse sand, medium sand, fine sand) and different numbers of treatment cycles (6, 8, and 10). The unconfined compressive strength (UCS), calcium carbonate content (CCC), and ultrasonic wave velocity of the bio-cemented sand columns were measured, and the influence of particle size on the cementation effect was examined using scanning electron microscopy (SEM). The results show that particle size significantly affects the UCS, CCC, wave velocity, and permeability coefficient of EICP-treated sand. After EICP treatment, medium sand exhibits the highest UCS and wave velocity. This is attributed to the fact that medium sand achieves an optimal balance between the pore-filling efficiency of calcium carbonate crystals and the permeability of the EICP solution. In fine sand, uneven CaCO? distribution caused by pore clogging hinders the uniform penetration of the EICP solution. In coarse sand, the large pore spaces make it difficult for the precipitated calcium carbonate to effectively cement the sand grains, resulting in limited cementation. The findings define the optimal particle size range for EICP technology and provide a theoretical basis for particle size selection in field-scale bio-cementation engineering.