［Objective］To determine the influence of sample characteristics such as sample size，sample shape， and sample type on the shear strength of granite residual soil.［Methods］A series of large-scale direct shear tests were carried out on granite residual soil by using a large-scale direct shear instrument. Based on the shear stress-displacement curve，shear strength and its parameters，the influence of the above-mentioned sample characteristics on the strength characteristics of granite residual soil was studied，and a recommendation was provided for determining strength parameters in practical engineering applications.［Results］Both the shear strength and its parameters of granite residual soil decreased first and then tended to stabilize as the sample size increased. The soil strength obtained under the optimal sample size could effectively overcome the influence of size effect on the test results. The shear strength and cohesion of square samples were smaller，while the internal friction angle was larger，than that of cylindrical samples. The non-uniformity of stress distribution speeded up the failure process of square samples. The shear strength of the original sample was significantly higher than that of the reconstructed sample，and the difference in shear strength between the original sample and reconstructed sample decreased with the increase of normal stress. The exponential function can be used to well describe the relationship between sample strength and sample size.［Conclusion］In practical engineering applications，large-diameter samples or small-diameter samples considering shear strength error should be used for large-scale direct shear test to obtain strength parameters that are better in line with practical engineering.