The fly-ash dam is used to store the fly ash discharged from the thermal power plant. A fly-ash dam is a special slope built with fly ash, and rainfall infiltration is an important reason to induce the landslide of this kind of slope. In this paper, the laboratory tests of different slope ratios and initial seepage fields under rainfall were carried out, aimed at studying the failure mechanism, failure mode, triggering mechanism, and influence factors for the slope instability of the fly ash dam slope under rainfall infiltration. The results show that: (I) Three failure mechanisms were found in the tests: sliding failure, runoff erosion, and flow-slide failure. Due to the low density of fly ash, runoff erosion is more likely to occur under rainfall. Differently from clay slope, flow slide is an important failure mechanism of fly ash slope under rainfall. (II) Local erosion damages caused by runoff erosion and flow slide are the important triggering factors of the fly-ash dam slope failure under rainfall. (III) Three failure modes were observed in the test: the overall sliding failure of the slope, the retrogressive landslide caused by multi-stage local sliding, and the gradual erosion failure of the slope (caused by the combined action of runoff erosion and flow slide). (IV) The slope ratio has an important influence on the failure mode. With the decrease in slope ratio, the failure mode evolves from sliding failure to flow-slide failure and runoff erosion failure. The greater the slope ratio, the more obvious the sliding failure characteristics; the lower the slope rate, the greater the runoff erosion damage. The existence of an internal seepage field in the slope intensifies the occurrence of flow slide.