Magnetostriction and spin polarization are two fundamental yet typically uncorrelated properties of magnetic materials. While magnetostriction reflects the strength of magnetoelastic coupling, spin polarization is determined by the spin-resolved electronic states near the Fermi level. Establishing a link between these two properties remains a significant challenge. In this work, we demonstrate that Fermi-level engineering in Co₂FeAl_xSi_1-x Heusler alloys provides an effective route to simultaneously enhance spin polarization, magnetostriction and temperature stability. First-principles calculations reveal that Co₂FeAl_xSi_1-x exhibits a strongly suppressed density of states in the minority-spin channel near the Fermi level across the entire composition range, characteristic of a minority-spin pseudogap. With increasing Al/Si ratio, the Fermi level shifts continuously from the lower edge of the pseudogap (x=1) to the upper edge (x=0), and resides close to the center of the pseudogap at x≈0.5. This electronic configuration leads to the highest calculated spin polarization and the maximum magnetostriction coefficient, both showing pronounced non-monotonic composition dependence. Polycrystalline samples with x=0-1 were synthesized to verify these predictions. Room-temperature magnetostriction measurements confirm that the saturation magnetostriction λ_s peaks at x≈0.5, in excellent agreement with theoretical calculations. Temperature-dependent measurements (80-300 K) show that the magnetostriction of all compositions follows the Bloch T^3/2 law, indicating that thermal excitation of spin waves is one of the primary mechanisms responsible for the temperature-induced reduction of magnetostriction. The extracted decay constant β is significantly reduced for intermediate compositions compared with the end-member alloys, and the low-temperature enhancement ratio R is also markedly smaller in this composition range, reflecting superior magnetostriction stability against thermal excitations. This enhanced stability can be attributed to the improved stability of spin polarization when the Fermi level resides near the center of the minority-spin pseudogap, which in turn weakens the impact of thermally excited spin waves on the magnetoelastic response. Further analysis indicates that the enhanced effectiveness of spin–orbit-coupling-induced modulation of magnetocrystalline anisotropy in this electronic configuration constitutes the key microscopic mechanism linking spin and lattice degrees of freedom, thereby enabling strong magnetoelastic coupling. These results establish an electronic-structure-based mechanism linking magnetostriction and spin polarization and provide a design principle for multifunctional Heusler materials with coupled magnetic and magnetoelastic performance.