Abstract: The flow field during the motion of an underwater explosion bubble is supposed as an inviscid, irrotational and incompressible ideal fluid to simulate the bubble motion in a free field using the boundary element method. An computational program is developed and in the calculation process, the numeric fair technique and elastic mesh technique are applied to avoid the numerical divergence induced by the mesh distortion. The error between the computed and experimental results is less than 10%. From the basic phenomenon of the underwater explosion bubble in the free field, the dynamic behavior of the bubble in the free field is investigated systematically by using the developed program. By analyzing the pressures of the fluid field in the different directions, it gains that the pressures in the directions of the bubble center movement and the jet attack are higher than those in other directions. It explains that the pressure loaded in the direction of the bubble jet attack is maximum and damages the underwater structure severely, showing the asymmetry of the bubble loading. Calculated velocity curves at the different positions in the fluid field reveal that the velocity of the retarded flow induced by the bubble motion decreases rapidly with the increase of the distance between the retarded flow and the bubble center, and the direction of the retarded flow reverses with the bubble expanding and collapsing. Attenuation and variation of the retarded flow are summed up.