To address the permeability deterioration and fatigue degradation of modified soil under high-moisture service conditions, this study investigated the impermeability and fatigue behaviour of modified soil under different lime contents, compaction degrees, moisture contents and curing durations through laboratory permeability tests and cyclic loading tests. The results indicate that the permeability coefficient of modified soil decreases with increasing lime content, compaction degree and curing age, while it increases with increasing moisture content. Under 28-day curing conditions, increasing the lime content from 4% to 10% reduced the permeability coefficient by 80.4%. Under cyclic loading, the accumulated fatigue failure cycles of modified soil increased with higher lime content and longer curing periods, but decreased with increasing peak load under the adopted step-loading protocol; the accumulated fatigue failure cycles increased by up to 1.89 times. In addition, an empirical peak load–cycle relationship was obtained only for the tested modified soil with 8% lime content under a 120-day curing period and the adopted step-loading protocol in this study, providing experimental support for engineering design and service-life evaluation under similar loading conditions.