Piles are widely used to reinforce slopes. Many uncertainties are involved in the design of slopes reinforced with piles. Probabilistic methods can be used to quantify the effect of uncertainties in slope stability analysis. However, evaluating the system reliability of slopes reinforced with piles is seldom reported. This paper investigates the system reliability of slopes reinforced with piles based on the shear strength reduction method. First, a finite difference model of a double layered slope reinforced with piles is established. Then, a multiple-design-point searching strategy based on HLRF-BFGS method is suggested to search for the representative slip surfaces of the slope. The system reliability is then studied based on the representative slip surfaces. The suggested method proved to be an efficient way to evaluate the reliability of the piled slopes. The optimal pile location and pile spacing are further studied. The results from this study will provide insights on the optimal design for slopes reinforced with piles