Abstract: Foam is one of the most commonly used conditioning agents for shield tunnelling in sandy strata; however, the seepage mechanism of foam-conditioned sandy soil under combined water-soil pressure remains unclear due to the high compressibility of foam bubbles, making it prone to gushing and potentially triggering ground instability. To address this, a series of permeability tests on foam-conditioned sandy soil are conducted using a self-developed novel external load-controlled permeameter, focusing on investigating the permeability characteristics and underlying mechanisms of foam-conditioned sandy soil under soil and water pressure. The results indicate that under low water pressure conditions, a significant excess pore water pressure is generated when the soil pressure is applied on the foam-conditioned sandy soil specimen. In contrast, no significant excess pore water pressure is observed under high water pressure conditions. The influence of soil pressure on the permeability of foam-conditioned sandy soil is related to the water pressure environment in which it is situated. Under low water pressure conditions, the permeability coefficient of foam-conditioned sandy soil first undergoes a "cliff-like" decline followed by a gradual decrease with increasing soil pressure, with a reduction exceeding one order of magnitude. The sensitivity of the foam-conditioned sandy soil’s permeability coefficient to soil pressure gradually diminishes with increasing water pressure. When the water pressure exceeds 125 kPa, the permeability coefficient no longer undergoes significant changes with increasing soil pressure. Water pressure causes foam bubbles to contract, whereas soil pressure merely deforms them without altering their volume. In terms of permeability, the soil pressure contributes to improving the anti-permeability performance of foam-conditioned sandy soil, while water pressure compromises the water-blocking capability of foam bubbles.