Inspired by observations of manta ray gliding, this study designed and evaluated a more biologically accurate pectoral fin bending model. We assessed its hydrodynamic performance using six-degrees-of-freedom (6-DoF) Computational Fluid Dynamics (CFD) simulations, which were validated by tethered water tunnel experiments. Key findings reveal that symmetric bending significantly impacts longitudinal stability, increasing the pitch angle to nearly twice that of the flat-wing model (80° model) but compromising gliding efficiency. During this symmetric motion, the lift-to-drag ratio (K) minimum point is significantly delayed as the bending angle increases, following a negative quadratic trend. Conversely, asymmetric bending triggers a sharp 3.5-fold increase in the roll angle (80° vs. 30° model) and produces significant lateral displacement. Importantly, “roll-induced yaw” was confirmed as the dominant mechanism for lateral control, contributing up to 88.5% of the lateral force in the 80° model, despite minimal changes in the yaw angle. These findings reveal the intrinsic trade-offs between fin deformation, gliding efficiency, and attitude control, providing a theoretical basis for active configuration optimization and control strategies for bionic gliders.