Abstract: Dispersive clay is a problematic soil that has too low erosion resistance. To improve its behaviors, we have conducted pinhole tests and crumb tests as well as compression and seepage deformation tests on the dispersive clay modified with different fly ash content. And SEM tests were used to analyze the microstructure and mechanism. The results showed that adding fly ash produced a significant improvement on dispersity, compressibility and permeability of this modified dispersive clay that depended on fly ash content, curing period, and compaction degree. Its dispersity and compressibility were both decreased with an increase in fly ash content or compaction degree; its compressibility was also decreased with an increase in curing period. The mechanism of all the improvement was a series of physical and chemical reactions between fly ash particles and soil particles. Adding fly ash was able to promote the formation of granular structure and reduce soil dispersity through hydration and hydrolysis reaction, ion exchange reaction, and hard condensation reaction. Thus, by mixing fly ash into dispersive clay, it is possible to transform it into non-dispersive clay. For engineering application, this study suggests a fly ash content of 4% - 10%, a curing period of longer than 7 days, and a compaction degree of 96%.