Abstract: This study aims to systematically quantify the influence of fin-hull geometric configuration on the propulsion performance of bionic undulating fins (Media and/or Paired Fin propulsion, MPF), addressing the lack of a unified analysis of geometric parameters across different bionic underwater vehicles in existing research. To this end, a universal parametric geometric model incorporating the hull and a pair of undulating fins was established, which innovatively introduces the ratio of fin width to hull width ( ) as the core dimensionless geometric parameter. Based on this model, high-fidelity CFD numerical simulations were conducted to analyze the propulsion performance and flow field structure of the vehicle under different values. The results indicate that γ has a nonlinear and significant impact on propulsion performance: an optimal range of values exists to maximize propulsion efficiency. Excessively small values lead to insufficient thrust generation, while excessively large values increase drag due to strong fin-hull interactions that induce flow separation. Furthermore, significantly modulates the magnitude of the pitching moment, imposing a critical constraint on the attitude stability of the vehicle. This study clarifies the design trade-off between efficiency and stability governed by the parameter. The established parametric model and the revealed influencing mechanisms provide a quantitative theoretical basis for the shape design of bionic underwater vehicles and lay a solid foundation for subsequent research on multi-parameter coupling optimization and self-propulsion performance.