Based on the upper plastic limit analysis theory, the kinematically admissible velocity field of a capped rigid pile is obtained through both numerical and model tests. According to the energy balance principle and unified strength theory, an equation is developed for the ultimate bearing capacity limit solution of the pile, in which the effect of the intermediate principal stress is considered. In addition, the variations of the pile ultimate bearing capacity with the cap size, soil cohesion and internal friction angle are analyzed through parametric analysis. It is found that the upper limit solution increases more significantly for the pile with considering the intermediate principal stress than those without. A reasonable range of the unified strength parameter is from 0.1 to 0.2 in calculating the rigid piles bearing capacity. With the increase of the cap size, the pile bearing capacity increases. When the ratio of cap diameter D to pile diameter d is greater than 2, the bearing capacity increases more significantly. With the increase of soil cohesion and internal friction angle, the bearing capacity increases drastically. By comparison with experimental results, it is found that the proposed method yields better results than the existing theoretical methods.