To calculate the ultimate bearing capacity of pile foundation in karst areas, a simplified analysis model is proposed, which fully considers the influence of the distance between the pile and the karst cave on the bearing capacity of pile foundation. Based on the upper bound limit analysis, the corresponding assumed failure mode is proposed. The triangular rigid block is used to discrete the failure mode, and the external force power and internal energy dissipation rate are calculated. Taking the ultimate bearing capacity of pile foundation as the objective function, the problem is transformed into a mathematical optimization model, which is solved by the MATLAB software. To verify the correctness of the present method, the calculation results of the present analysis are compared with those from the upper bound finite element limit analysis. Finally, the influence of material parameters and geometric parameters on the ultimate bearing capacity of pile foundation is analyzed. The parameter analysis shows that with the increasing distance between the pile and the karst cave, the influence of the karst cave on the bearing capacity of the pile tip becomes smaller and smaller. When the distance between the pile and the karst cave exceeds a certain value, the ultimate bearing capacity of the pile tip will not be affected by the karst cave.