Apteronotus albifrons possesses an elongated anal fin that enhances its cruising efficiency and maneuverability in complex environments. This study utilizes a three-dimensional numerical model in conjunction with dynamic mesh technology to analyze the hydrodynamic properties of a large-amplitude undulating fin under varying flow velocities and oscillation amplitudes. Based on the “differential first, then integral” approach, the thrust coefficient and propulsion efficiency of the undulating fin are evaluated using computational simulation methods. The results indicate that as the advance ratio increases, the thrust coefficient gradually decreases, becoming negative when the advance ratio reaches 1. At an advance ratio of 0.75, the average propulsion efficiency peaks at approximately 65.8%. Furthermore, larger oscillation amplitudes contribute to an increase in the thrust generated by the undulating fin, significantly enhancing its overall operational performance. These findings provide essential methodologies and theoretical support for the future optimization and design of undulating fins.