The Loess Plateau features a fragile ecological and geological environment, with landslide disasters occurring frequently. Among the triggering factors of these disasters, earthquakes and rainfall are the most prominent. To better understand slope failure mechanisms under these conditions, this study performed comparative shaking table tests on rainfall and non-rainfall steep loess slopes. The test results indicate that earthquakes facilitate rainfall infiltration to 0.54 of the slope height. Loess layers above the infiltration front critically influence the landslide's dynamic response and failure mechanism. Under seismic loading, the pore water pressure (PWP) within this layer increases to a positive value, thereby reducing the loess's effective stress. The acceleration amplification factor (AAF) of the rainfall slope is approximately 24 % higher than that of the non-rainfall slope, indicating that rainfall exposes the slope to greater inertial forces. Further analysis using the Hilbert-Huang transform revealed that rainfall altered the slope's stiffness, enhancing its capacity to absorb high-frequency (10-50 Hz) seismic waves and thereby rendering it more susceptible to seismic impact. Additionally, significant seismic subsidence was observed at the crest of the rainfall slope, and the combined added and inertial forces induced tensile cracks and large displacements in the soil at the infiltration front. This study clarifies the mechanisms driving loess landslides under combined rainfall-seismic effects.