The highway slope is prone to landslide due to rainfall infiltration, which causes great harm to the highway safety. Taking a high filling project in mountainous area as the engineering background, stability of reinforced high slope was studied with the consideration of rainy. A model was established using FLAC software to analyze the reinforced high rock slope. The variation patterns of slope displacement, pore water pressure, and slope stability coefficients under different rainfall conditions were investigated. The results showed that during the process of rainfall infiltration, the displacement of the strongly weathered rock stratums of the rock slope increased, but the overall stability of the slope was maintained. During the rainfall, the prestress on the anchor cables and bending moment of frame beams both continuously increase with the continuous rainfall. After the rainfall, the pore water within the slope seeped down towards the foot of the slope along the interfaces of the rock stratums. The increase of rainfall time was detrimental to the stability of the high rock slope.