Abstract: In the construction of water conservancy and hydropower projects in western regions, ultra-deep alluvial sand and gravel deposits are commonly encountered. These formations are characterized by complex genesis, significant structural heterogeneity, and challenges in obtaining reliable in-situ mechanical parameters at great depths. Conventional pressuremeter testing in such ultra-deep sand and gravel layers often suffers from membrane rupture and incomplete test execution. To address these issues, a field pre-bored pressuremeter test was conducted in a western region site featuring a sand and gravel deposit exceeding 200 meters in thickness. The test employed a steel-wire-reinforced rubber membrane combined with an externally mounted slotted steel protective casing. Based on the acquired pressure-volume (P-V) curves, a systematic analysis was performed to evaluate the deformation behavior, lateral pressure shear modulus, foundation bearing capacity, and initial tangential modulus of the sand and gravel layers at various depths under in-situ stress conditions. Additionally, different methodologies for determining shear strength parameters were comparatively assessed. The findings indicate that the modified pressuremeter technique is well adapted to ultra-deep sand and gravel conditions, yielding P-V curves with clearly defined three-stage characteristics. Overall, the ultra-deep sand and gravel layer exhibits a high lateral pressure shear modulus and substantial foundation bearing capacity, reflecting high stiffness and low compressibility. Mechanical properties generally increase with depth, although notable fluctuations occur in response to local variations in stratigraphic composition. Under overburden pressure, the initial tangential modulus is consistently high, though significant variability is observed across different boreholes and depth intervals. Discrepancies exist among shear strength parameters derived from different analytical methods based on the same pressuremeter data. These results provide valuable insights for the selection of geotechnical parameters and foundation design in water conservancy and hydropower projects situated on ultra-deep alluvial deposits in western China.