In dike engineering projects for rivers and lakes, continuously heavy rainfall will lead to a significant change in the water level, which may cause safety hazards to dikes. To solve these problems, a water-gas two-phase unsaturated seepage model was used to analyze the saturated-unsaturated seepage of a dike under the coupled conditions of rainfall infiltration and water level change. Finite element simulation was performed to study the unsteady seepage in the processes of rainfall infiltration, water level rise in front of the dike, and coupling of continuously heavy rainfall and the water level rise. On these bases, anti-sliding stability analysis of the dike slope was conducted, with consideration of the effects of the dike water level changes, and the gas phase and matric suction during rainfall on the slope stability. The results show that, compared to the rainfall infiltration or the water level rise, the coupled conditions of rainfall and the water level rise will cause the seepage and slope stability of the dike to become more complex. Considering the gas phase and matrix suction will be helpful to slope stability analysis.