Abstract: For an underwater tunnel driven by a slurry shield machine, the formation of an airtight filter cake is key to the success of hyperbaric intervention under complex geological conditions. Great differences exist among filter cakes formed under different conditions, yet the effects on airtightness are unclear. In this paper, a formation and airtightness test was performed on 9 different filter cakes by a self-made testing unit, and the effects of porosity and thickness of the filter cake on airtightness (airproof time and air pressure limit) were analyzed. The results show that: 1) porosity is the major influence affecting airtightness, and performance is better with less porosity; 2) filter cakes formed in the same stratum have similar porosities, and airtightness can be improved by increasing the thickness of the filter cake to acertain extent; 3) nearly 95% of the filtration water emerges from the compression of the filter cake, while the rest is generated by air penetrating the pores and stratum compression; therefore the three stages of filter-cake compression, air penetration into pores and water flow into the filter cake are adopted to provide a better understanding of the process of the filter cake.