Natural rainfall and atomization rain will make infiltration into slope with a certain time process, which influences the slope stability in the downstream of high dam. The influence manifests in two aspects, on the one hand, the increase of water content increases the bulk density of the soil, resulting in an increase in the sliding force of slope; on the other hand, the soil moisture content increases, which makes the shear parameters of slope reduced. Engineering events caused by rainfall infiltration occur frequently. In the study, an indoor artificial rainfall physical model test was designed to study the process and law of water diffusion and the expansion process of transient saturation zone in the slope during rainfall infiltration of sand slope. The safety factor of slope was calculated to analyze the effect of rainfall infiltration process on slope stability. The test results showed that a transient saturated zone was formed firstly in the surface of the slope. As the rainfall continued, the transient saturation zone gradually expanded. The greater the rainfall intensity, the larger the transient saturation zone formed by the rainfall process, and the greater the reduction of the slope safety factor. The empirical formula of the water infiltration depth along with the rainfall duration and rainfall intensity was obtained. Increasing the rainfall intensity would make the water content of the test sand closer to the saturate water content, but the sand wouldn't completely saturate. The effect of rainfall infiltration in slope not only occurred during the rainfall process. After the rainfall stopped, the water infiltration process kept existing, the stability of the slope continued to decrease, and the water did harm to the slope in a certain delay time. In the case of rainfall intensity of 144 mm/h, the stability safety factor of the slope decreased to a minimum of 1.196 at 6 h, with a maximum decrease of 38.2%.