Abstract: In this study, a three-dimensional simulation of a fish-like body swimming in a narrow channel with non-slip walls is carried out using the immersed boundary method. The fluctuation direction of the body is parallel to the walls. The results show that at the same Reynolds number, as the channel becomes narrower (i.e., the distance between the upper and lower walls decreases), the swimming speed of the fish-like body increases while its power consumption decreases. At a higher Reynolds number (Re=2600), the power consumption is consistently lower than that of free swimming within the range of wall distances studied. However, at a lower Reynolds number (Re=410), there are distinct zones compared to free swimming. Furthermore, the effect of the Reynolds number is investigated. At the same channel height, as the Reynolds number decreases, the swimming speed of the fish-like body decreases but remains higher than that of free swimming, and its power consumption increases but stays lower than that of free swimming. Finally, to explore the underlying physical mechanisms, we analyze the vortex structures and vorticity field generated by the fish-like body. We find that the wall effect enhances the intensity of the wake jet, resulting in increased swimming speed and decreased power consumption. Conversely, a reduction in the Reynolds number leads to a decrease in the intensity of the wake jet and a change in its mode, which reduces the swimming speed and increases the power consumption of the fish-like body.