<p>Existing studies on fish swimming have generally examined the effects of the surrounding flow field under stationary conditions using a uniform incoming flow. Moreover, most research has focused on flow phenomena around individual fish or specific regions of the fish body. However, an increasing number of studies have demonstrated that group locomotion not only reduces energy consumption but also provides sociological benefits, including predator avoidance and enhanced predation success. In this study, we employ an overset mesh technique—implemented through the secondary development of a user defined file in Fluent—to investigate the influence of passive hydrodynamics on the self-propelled swimming of fish under a uniform incoming flow. This study primarily focuses on elucidating the mechanisms underlying fish self-propelled swimming and the relationships among tail-beat frequency, incoming flow velocity, and swimming performance, while also examining the hydrodynamic variations in both juxtaposed and tandem fish pair configurations. The study demonstrated that passive hydrodynamics did not contribute to speed gain in side-by-side swimming fish. As the inter-fish spacing decreased, the negative effects became more pronounced, although phase variations enhanced propulsion. In contrast, tandem swimming fish benefited from hydrodynamic interactions, with the upstream fish experiencing greater advantages than the downstream fish; however, the influence of wake vortices occasionally disrupted these benefits. However, due to intermittent interference from the wake vortex, these hydrodynamic benefits are not consistently sustained. In this study, we examine the variations in key parameters during the swimming process of fish schools, which is essential for elucidating their underlying swimming mechanisms.</p>