When dealing with special soil, it is very important to choose environment-friendly and low cost materials with good properties for the strengthening. In order to study the effect of basalt fiber and nano-iron oxide on the mechanical properties of granite residual soil, samples with different dosages and mixing methods were prepared for the consolidation and undrained triaxial shear test, and the micro-mechanism of the processed samples was analyzed by scanning electron microscope (SEM). The experimental results show that the shear strength of soil samples first increases then decreases with the increase of the dosage of basalt fiber, and peaks when the dosage reaches 1%; whereas the shear strength of soil samples increases monotonously with the increase of nano-iron oxide content. Both the single agent and composite agents methods can improve the cohesion of the soil, but not the internal friction angle. In addition, the effect of composite modified soil sample on soil shear strength is more obvious than that of single modified soil sample. The improvement of soil shear strength is contributed by the interlocking mesh between the basalt fiber and soil, and the filling of the gaps between soil particles by nano-iron oxide. The method of composite soil sample modification can effectively improve the shear strength characteristics of granite residual soil.