Abstract: To investigate the rainfall infiltration characteristics of heterogeneous cobble-boulder mixed soil under natural slope conditions, an in-situ artificial rainfall simulation test is conducted on a representative nearly 200 m² slope area in the reservoir region of the Zala Hydropower Station, Tibet. Three test zones with distinct geological conditions are subjected to uniform rainfall simulation (intensity: 27 mm/h; total rainfall: 250 mm and 70mm), while monitoring dynamic changes in soil moisture content and pore air pressure at varying depths. Key findings include: (1) Spatial heterogeneity of cemented layers and cemented aggregates induces strong heterogeneity in the soil matrix. Although cemented layers exhibit partial impermeability, local preferential flow patterns develop in weak zones under sufficient rainfall. (2) In the cobble–boulder mixed soil with a relatively simple layered structure, the dynamic variation of soil moisture exhibits a depth-dependent hysteretic behavior, with shallower layers responding earlier, rising more rapidly, and maintaining the peak for a longer duration. Post-rainfall moisture decay follows an analogous depth-governed behavior. (3) For the first time, a two-phase (water-air) oscillatory response controlled by cemented layers is captured. Cemented layers trigger shallow transient saturation zones, where pore air pressure undergoes four-phase oscillation (rapid surge–steady state–abrupt decline–recovery). After the cemented layer is removed, infiltration becomes predominantly vertical, with negligible pore air pressure fluctuations. This study demonstrates that rainfall infiltration in cobble-boulder mixed soils is governed by heterogeneous structures, where cemented layers exert dual control on slope stability by suppressing deep recharge and inducing shallow saturation.