Integral abutment jointless bridges (IAJBs) offer superior structural robustness, seismic performance, and long-term durability owing to the elimination of expansion joints and bearings, making them particularly suitable for mountainous regions. However, when such bridges are constructed on sloping terrain, the abutment-pile system is subjected to asymmetric soil constraints—the so-called "slope effect"—which complicates the soil-abutment-pile interaction (SAPI). To quantify this effect, pseudo-static cyclic tests were conducted on an integral abutment-reinforced concrete (RC) pile system embedded in a layered clay-sand slope, taking the ratio of the distance from the pile side to the slope crest to the pile diameter (b/d) as the key parameter. Two configurations were tested: APSD-1 (b/d = 0.0, level ground) and APSD-2 (b/d = 2.0, 30°slope). The results show that the slope effect significantly weakens the lateral resistance and amplifies the asymmetric response of the system. Compared with the level-ground case, the peak lateral force in the slope-facing direction decreased by 33.1% and the overall system stiffness degraded by 24.8%, while the plastic hinge in the pile developed to a greater depth. The earth pressure behind the abutment exhibited similar distribution patterns in both configurations, but its magnitude in APSD-2 was approximately 4.6% lower at large displacements. The average soil reaction on the pile decreased to 0.932 times that of the level-ground case, with a pronounced asymmetry in the embedment depths of the maximum reaction between the backslope and slope-facing directions. In addition, the equivalent viscous damping ratio in the slope-facing direction was reduced by about 5% on average. These findings clarify the integrated interaction mechanism and asymmetric failure characteristics of the abutment-pile-soil system in sloping terrain and provide essential experimental evidence for the seismic design and performance assessment of IAJBs in mountainous regions.