To clarify the mechanism of slope soil spalling induced by rainfall in shallow permafrost zones, a typical roadbed slope of the G214 lines in a permafrost area is taken as an example. The spalling process of slope surface soil under different slope angles and erosion velocities was simulated using the particle flow and equivalent rainfall method. Detailed exploration was conducted from aspects such as slope shape, particle transport law, force chain evolution law, and spalling amount. The results show that after the slope soil in the shallow permafrost zone is washed by rainwater, only the shallow part of the slope surface displays grooves. A slope shape coefficient (τ) was proposed to quantitatively characterize the degree of concave and convex slope shape. The overall change trend of slope shape is summarized as follows: straight and flat (1. 0≤τ < 1. 40) , rough and uneven (1. 40 ≤τ < 1. 50) , and concave-convex uneven (1. 50 ≤τ) . The evolution of the contact force of soil particles mainly occurs in the early stage of rainfall erosion, and the evolution characteristics of the contact force are not obvious during the erosion process. As the number of cycles increases, the larger the erosion velocity, the fewer the overall number of contacts, but the number of contacts in the final stable state is almost the same. Additionally, the loose structure of shallow soil on slopes under freeze-thaw cycles is the fundamental reason for the susceptibility of slope soil in permafrost zones to erosion and spalling by rainwater, leading to the formation of gullies on the slope surface.