The synergistic propulsion mechanism of the rear dual hydrofoils (abbreviated as "rear dual foils") in a biomimetic sea turtle fin-like quadruped hydrofoil device remains unclear. This study established a rear dual-foil propulsion dynamics model based on its motion principles. Computational fluid dynamics (CFD) numerical simulations were employed to investigate the hydrodynamic performance. The analysis focused on the effects of bending deformation, hydrofoil inclination angle, and hydrofoils distance on propulsion performance. Key parameters regulating thrust in flexible hydrofoils were quantified. Coupled analysis of vorticity and pressure fields revealed the underlying fluid dynamic mechanisms. Bending deformation enhances vortex shedding. Reduced hydrofoils distance optimizes pressure difference distribution. Under identical motion patterns, bending deformation strengthens vortex generation and shedding. Flexible dual foils (α=0.06) showed an average thrust increase of 139.3% compared to rigid dual foils (α=0). Changes in the inclination angle between the two foils had limited impact on thrust. Reduced inter-foil spacing effectively increased surface pressure difference. The model with L=2l/3 achieved approximately 16.8% higher average thrust than the model with L=4l/3.Increasing bending deformation and decreasing inter-foil spacing both significantly improve propulsion performance. Comprehensive optimization of bending deformation and inter-foil spacing elevates average thrust by 187.5%.