The coupling effect of rainfall and earthquake induces failure and sliding of loess slopes, which is a mutually reinforcing relationship. A new instability model for loess slopes has been proposed, which solves for external force work and internal energy dissipation, and based on functional principles, obtains upper bound solutions for different variables. The results indicate that as the height ratio of the broken line slope increases, the upper bound solution for the sliding and failure of the loess slope also increases, that is, the higher the lower slope, the more stable the slope. When the upper slope angle of the folded slope decreases and the lower slope angle increases, the upper bound solution also increases, that is, the stability of the loess slope increases with the increase of the lower slope angle. This study reveals that under strong earthquake action, the sliding surface of loess slope instability and failure is located below the wetting line. When the coupling effect between rainfall and earthquake reaches a certain coupling value, the loess slope fails and slides along the wet line. When earthquakes are weak and rainfall is strong, landslides are located above the wet line. This research can provide a reliable theoretical basis for the design of stable support for loess slopes.