Abstract: To elucidate the impact of spar design parameters on the sectional stiffness and on the aeroelastic stability of wind turbine blades, this study develops a high‑fidelity model of the NREL 5 MW blade using the Variational Asymptotic Method (VABS). The complete 6 × 6 stiffness matrices are computed for 27 spanwise sections, and the effects of spar thickness and width on each stiffness component are systematically evaluated. Research results indicate that: a 10 % increase in spar thickness raises the axial stiffness K₃₃ by approximately 6.8 % on average, while widening the spar to 800 mm enhances the flapwise stiffness K₅₅ by about 18 % on average. Among off‑diagonal terms, the tension‑bending coupling K₃₅ shows the highest sensitivity to thickness variation, with a maximum change of 30 %. Subsequent aeroelastic stability analysis identifies the tension‑bending coupling term K₃₄ as the decisive factor for the flutter stability, contributing solely about 6.3 % to the critical speed margin. The full 6 × 6 stiffness matrix model achieves roughly 2.1 % higher stability than that of the conventional 4 × 4 model. By quantifying how spar parameters govern the coupled stiffness terms, this study achieves the rapid regulation of the coupling stiffness and the aeroelasticity of the blade in the main beam structure design. At the same time, it provided a design basis for the structural optimization considering the stiffness characteristics and aeroelastic stability of wind turbine blades.