In order to study the effect of fly ash content on the permeability of soil-cement, fly ash soil-cement test blocks with different curing agent ratios were prepared by adding water glass into fly ash soil-cement to stimulate fly ash activity. Then penetration test and electron microscopy test were carried out at different curing ages. The test results show that the permeability coefficients of the two cement soils are significantly reduced with the increase of curing age. Compared with pure soil-cement, fly ash soil-cement with 60% cement content has a higher permeability coefficient before curing for 14 days, however, the permeability coefficient after curing for 28 days becomes lower. The backpressure difference of permeability test has a great influence on the samples with short curing age. From the microscopic point of view, there is a clear correlation between the internal pore structure of the cement soil samples and the permeability coefficient.