Rainfall-induced loess landslides are widespread across the Loess Plateau, especially in the middle and south region. The rainfall infiltration characteristics of slopes are vital for slope instability mechanisms research and for hazard warning and prevention. To explore the rainwater infiltration and hydrological characteristics of loess and evaluate the preferential contribution of infiltration, with a focus on the Luoyu Valley in Tianshui city as a case study. A slope was selected for conducting artificial rainfall tests, which were monitored via sensors and electrical resistivity tomography (ERT). The sensor and ERT results indicate that preferential infiltration widely occurs in the loess slope, affecting the distribution of rainwater, the shape of the wetting front, and the rate of infiltration. On this basis, the variations in hydraulic factors in the loess slope and water infiltration direction are revealed under the impact of preferential infiltration. Numerical simulations were subsequently conducted to explore infiltration characteristics in uniform zones and preferential channels. In the slope, under pressure infiltration (ponding water) conditions, preferential infiltration occurs within fissures and affects the whole morphology of the wetting front. Our results emphasize the pivotal role of preferential infiltration in loess slope infiltration, which is of great theoretical value in landslide stability calculations.