<p>It is crucial for practitioners to rationally calculate the lateral earth pressure on multi-row long piles in soil slopes considering the depth of rainfall infiltration in some mountainous projects. Based on the mobilized soil strength parameters associated with shear strain along the potential slip surface, this study proposes a simplified analysis method for estimating both non-limit and limit earth pressures on piles. The approach integrates an inclined slice technique, the Mohr–Coulomb yield criterion, and the beam-on-elastic-foundation model. The proposed method can solve the upslope thrust force on the rear pile, the downslope resistance on the front pile, the driving force between adjacent piles, and the most potential slip surface in multi-row piled slopes. Also, this method can quantitatively exhibit the inflection point at the interface between the rainfall infiltration zone and its lower zone in the piled slope. A series of model tests are conducted to verify the proposed method, which reveals that the earth pressures on the rear and front piles increase by 3.33% and 7.52%, respectively, when the rainfall infiltration depth reaches 0.2 times of the filling height. The error between the proposed results and the observed results is less than 16%. The proposed method can adequately reflect the effect of multiple factors on the net thrust on piles, such as soil properties, rainfall infiltration depth, pile top lateral displacement, and pile row spacing.</p>