Abstract: The fly ash dams in the storage field of coal-fired power plants are mostly in unsaturated state. The surface ash body experiences the change from unsaturated state to saturated one during floods, which will greatly affect the slope stability. However, the experimental researches on the unsaturated characteristics of fly ash in this process are rarely reported. A coal fly ash is chosen as the research object, and the related unsaturated infiltration test device is developed. The systematical tests are then carried out on the infiltration characteristics of a fly ash dam under different flood water-head conditions such as 1.0, 2.5, 3.3 and 5.2 m. The test results show that the average infiltration coefficient of forward edge surface for unsaturated fly ash is 1.2×10^-3 cm/s, while the infiltration coefficient of saturated surface is A×10^-4 cm/s level, and it is closer and closer to that of弹the forward edge surface with the increasing flood water-head. Based on this, a method for estimating the saturated area in the fly ash dam is proposed, that is, the infiltration coefficient of saturated surface under low flood water-head can be obtained from the test results, while the coefficient of forward edge surface can be used to estimate the saturated area when the flood water-head is larger than 10.0 m. The accuracy of the proposed method is verified by comparing with the numerical results. As a result, based on the stability analysis results of upstream slope of a typical fly ash dam considering flood infiltration, the analysis method based on the estimation of saturated zone can be better reflect the actual stability of the upstream slope, and the research results may provide reference for the unsaturated seepage and stability analysis of fly ash dams.