At present, straw fibers have been widely applied in improving material performance in civil engineering. However, the mechanical behavior of granite residual soil reinforced with rice-straw fibers remains unclear. Direct shear tests were conducted to investigate the strength characteristics of granite residual soil reinforced with rice-straw fibers. By varying the fiber length and content, the evolution of shear strength, internal friction angle, and cohesion in fiber-reinforced residual granite soil under different scales was examined. The major findings are as follows.Under various vertical stresses, for small specimens of saturated granite residual soil reinforced with rice-straw fiber, the shear strength increases with fiber content; the increase is more pronounced at lower stresses. Under 50 kPa, the shear strength was raised by approximately 57% when 5% fiber content was added, compared to the unreinforced soil. Higher shear strength is achieved with longer fibers.Across different fiber lengths, the trend of strength parameters with fiber content is consistent: as content increases, cohesion increases. With 2 cm fibers at 5% content, cohesion reaches about 7.2 kPa, whereas without fibers it is only 2.2 kPa. The internal friction angle is hardly affected by fiber content. Similarly, for various fiber contents, the influence of fiber length on strength parameters is nearly consistent: longer fibers lead to higher cohesion. At 1% fiber content, the cohesion with 3 cm fibers is about 5.5 kPa, whereas with 1 cm fibers it is only 3.7 kPa; the internal friction angle again shows negligible dependence on fiber length.For large (non-saturated) specimens of fiber-reinforced granite residual soil, the variation trends of shear strength parameters with fiber content are similar to those in small specimens. However, under comparable conditions, the strength indices of the large specimens are significantly higher. Without fiber, the cohesion and internal friction angle of the large specimen are 5.3 and 2.7 times those of the small specimen, respectively; at 1.0% fiber content, the corresponding ratios are 5.7 and 2.9 times.