Abstract: The mechanical properties and scale effects of dump materials are critical factors affecting the reliability of slope stability analysis in open-pit coal mines. To address the discrepancy between laboratory test materials and in-situ raw materials, which may lead to deviations in shear strength parameters, a systematic investigation is conducted based on the dump of the Shengli Open-pit Mine. Laboratory direct shear tests and in-situ direct shear tests are combined to examine the shear deformation behavior and strength parameter differences of granular materials under different gradation conditions and testing scales, and their influence on slope stability calculations is further evaluated. Laboratory direct shear tests on reconstituted specimens and in-situ direct shear tests on raw materials are performed to analyze the shear stress–shear displacement relationships and shear strength parameters of materials with different lithological combinations. In addition, particle size analyses are conducted to obtain the average gradation curves of in-situ raw materials and laboratory test materials, and the uniformity coefficient and curvature coefficient are calculated to quantify gradation differences. The results indicate that in-situ raw materials exhibit a wider particle size distribution and better gradation continuity, with higher uniformity and curvature coefficients than laboratory test materials, reflecting a certain reduction in structural integrity after laboratory reconstruction. In laboratory direct shear tests, the shear stress–shear displacement relationships show increased peak shear stress and enhanced shear deformation with increasing normal stress. The overall trends obtained from in-situ direct shear tests are consistent with those from laboratory tests; however, higher shear stress levels are observed in the in-situ tests under the same normal stress conditions. Linear fitting of shear strength parameters demonstrates that both cohesion and internal friction angle derived from in-situ tests are greater than those obtained from laboratory tests, indicating that particle interlocking, preservation of the original structure, and testing scale exert significant influences on shear strength. Slope stability analyses based on different strength parameters reveal that safety factors calculated using in-situ test parameters are generally higher than those obtained from laboratory parameters, showing noticeable differences in numerical results. These findings indicate that incorporating in-situ direct shear test results to revise laboratory-derived parameters is necessary for improving the rationality and engineering applicability of slope stability evaluations for dump slopes in open-pit coal mines.