The overset mesh method was used for numerical calculation of the movement process of a moving object in a pipe driven by a pump, the appropriate set solving parameters, such as mesh scale, iteration, time step, etc., were determined by reducing the fluctuation of the force applied to the object, the movement process of the object in the pipe from stillness to linearly increased revolution speed was explored, the movement process and movement law of the moving objects with different diameters were analyzed, the change in flow field of each moving object at the gap between it and the pipe wall was compared, and the effect of the gap on the fluid force on the object was analyzed in this paper. The simulation results showed that the forces on and movement fluctuations of moving objects at different gaps were different, and the gap sizes also significantly affected the fluctuations of fluid forces on the moving objects.