Abstract: The airtightness of a filter cake is critical concerning face stability during intervention under compressed air pressure in a slurry shield. However, the influence of slurry properties on the airtightness of filter cakes has not been identified, and on-site slurry preparation can become a difficulty that requires attention. Slurries with different properties were prepared by means of changing the additive dosage of CMC, and a self-developed laboratory test system for filter-cake air tightness was adopted to conduct filter-cake formation and airtightness tests. The results indicate that a filter cake has different airtight times at different air pressures and will be broken down by compressed air when the airtight time is over; due to high viscosity induced difficulties of slurry penetrating into the formation, the filter cake becomes thinner and is easily broken down, with its airtight time first increasing and then decreasing with increasing slurry viscosity. With respect to the Nanjing Weisanlu river-crossing tunnel project, it is recommended that the slurry density should be within 1.15~1.20 g/cm3, the CMC additive dosage should be 0.03%~0.1% and the funnel viscosity should be 40~50 s for slurry shield intervention under compressed air pressure in the pebble gravel stratum.