As a paradigm of efficient marine locomotion, tuna (Thunnus spp.) exhibits remarkable swimming performance that is intrinsically linked to their unique body stiffness modulation mechanisms. This study employs a coupled Peridynamics-Immersed Boundary-Lattice Boltzmann Method (PD-IB-LBM) framework to investigate the hydrodynamic performance of stiffness-modulated fish bodies systematically. A novel chordwise-weighted dimensionless stiffness parameter is introduced, which exhibits particular sensitivity to caudal stiffness variations and effectively characterizes the influence of stiffness distribution on propulsive performance. The research reveals that optimized stiffness gradients can significantly enhance propulsion characteristics. At a stiffness modulation parameter of decay rate value-0.2, the system demonstrates optimal performance, with simultaneous improvements in both swimming speed and propulsive efficiency. While moderate increases in caudal flexibility reduce absolute velocity, they substantially decrease the cost of transport (COT), providing new insights into the swimming strategies evolved in nature. By establishing quantitative relationships between swimming speed, efficiency, and stiffness parameters, this study provides a theoretical foundation for stiffness optimization in bio-inspired propulsion systems. Particularly, the proposed caudal stiffness modulation strategy offers significant implications for developing next-generation variable-stiffness underwater propulsors.