The study of the effect of free surface on the propulsion efficiency enhancement in fish body motion and its significance in biomimetic fish design is of great importance. This research establishes a numerical model of fish-shaped bodies and their oscillating tail models based on the NACA0012 airfoil. The motion of the fish body is controlled using field functions and dynamic mesh technology, and numerical analyses of the propulsion efficiency of the oscillating fish body under a free surface are conducted using the commercial software STAR-CCM. The study analyzes the impact of free surface distance on propulsion efficiency and reveals the structural distribution characteristics of the flow field under the influence of the free surface. The results indicate that oscillatory propulsion near the free surface can effectively reduce drag, with the drag decreasing as the free surface distance gets closer, although it is also influenced by the motion posture. When the fish body is very close to the free surface, the reactive effects of the free surface cause irregular variations in propulsion efficiency; however, the overall motion characteristics are still maintained.