This study performed a three-dimensional (3-D) finite element numerical analysis on a retaining piles stabilized slope of Ali-Shan Wu-Wantzy landslides (or Ali-Shan landslides) at Chia-Yi County to inspect the slope stability and in-situ lateral earth pressure acting on the piles. In addition, two groups of model slope, namely, colluvium slope (homogeneous slope) and colluvium-bedrock slope (heterogeneous slope) were set up for a series of numerical experiments on the design parameters of retaining pile to investigate their influences on the slope stability and the mechanical behaviors of retaining piles. The analysis results indicate that the pile length turns to be influential to the factor safety if the retaining piles are installed at the central area of the homogeneous slope. In addition, the mobilization of maximum bending moment of retaining pile in steeper heterogeneous slope is greatly influenced by the penetration depth of pile into bedrock.