The embedded length of anti-slide piles reinforcing slopes is analyzed by three-dimensional elasto-plastic shear strength reduction finite difference method. The effect of embedded pile length on safety factor and pile behavior, and the effects of the pile spacing, pile head conditions, bending stiffness and soil style on pile length and pile behavior are analyzed. The results show that the pile spacing and the pile head conditions have significant influence on the critical pile length. The critical pile length is seen to increase with decreasing pile spacing, and smaller pile spacing tends to increase the integrity of the piled slopes. A theoretical analysis of the slip surface is also described, and the slip surface determined by the pressure on piles considering the influences of both soil and the piles of slopes is in agreement with previous researches.