The pile leg detection of offshore structure is of great significance for structure safety. The maneuvering performance of underwater robot with oscillating propulsion is excellent, and it can be used as one of the main detection means, but the hydrodynamic performance of its two-dimensional flapping foil is disturbed by the vortex induced by pile leg in actual operation. The simulation of pile leg is performed with the circular cylinder, the numerical simulation of hydrodynamic performance of flapping foil in a flow field of flow around a circular cylinder is performed with overlapping grid technology, and the influence of main parameters such as offset between flapping foil and pile leg center, distance between flapping foil and pile leg, and initial phase of flapping foil on hydrodynamic performance of flapping foil is analyzed. The results show that the proper increase of offset between flapping foil and pile leg center can improve the thrust of underwater robot with oscillating propulsion.