Conventional polymeric systems face significant challenges in maintaining performance under high-temperature, high-salinity reservoir conditions due to limited thermal and saline stability. To address this critical limitation, a hydrophobically modified biopolymer (HWLG) was synthesized via etherification of Welan gum (WLG) with 1-bromooctadecane, introducing alkyl grafts to create hydrophobic microdomains. Comprehensive structural characterization was performed using Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy (NMR), gel permeation chromatography (GPC), thermogravimetry analysis (TGA), and scanning electron microscopy (SEM), confirming successful alkyl incorporation. Rheological evaluations demonstrated HWLG’s concentration-dependent pseudoplasticity, achieving a viscosity of 1423.2 mPa·s at 4000 mg·L−1, which was about 3.4 times that of WLG at 70 °C. The HWLG solution showed superior temperature and salt resistance in comparison with unmodified WLG, due to hydrophobic association-driven network formation. Particularly in formation water, HWLG showed a better long-term thermal stability, retaining 64.5% viscosity after aging 50 d at 70 °C, compared to WLG’s 39.6% retention. Core flooding experiments validated HWLG’s EOR efficacy, delivering 22.6% incremental oil recovery versus WLG’s 13.7%, driven by enhanced mobility control. The integration of hydrophobic functionality endows HWLG with exceptional thermosaline stability, adsorption capacity, and viscoelasticity, positioning it as a robust candidate for high-temperature, high-salinity reservoir flooding applications.