Slurry circulation system is an important part in slurry shield tunneling. The mucks conveyed by the slurry can be separated from the slurry by the screening machine. The screening efficiency of the screening machine has great influence on the property of the slurry, and therefore has close relation with the slurry preparation cost. When a large slurry shield tunnels through fine silty sand strata, in each boring cycle, the density of the slurry falls from 1.12 g/ cm3 to 1.06 g/cm3. Therefore, at the job site, a large amount of bentonite is added to increase the density of the slurry. In the paper, the fine silty sand strata is analyzed and relevant calculation is made; the influence of the screening efficiency on the slurry density is studied and comparison is made between the study result and the measurement data. The study shows that the fine particles with less than 75 μm size in the strata are helpful to the increasing of the slurry density and therefore should be fully used. Therefore, it proposed that the screening efficiency of the screen machine should be well controlled so that the fine particles in the strata can be kept and used to improve the slurry density. 