The invention discloses a large-scale in-situ triaxial shear test device and a method thereof, and belongs to the technology of mechanical property test of soil in the field of geotechnical engineering. The device comprises a sample (10), a confining pressure system (20), a vertical pressure system (30), a counterforce system (40) and an automatic data acquisition system (50); the sample (10) is arranged in the confining pressure system (20); the vertical pressure system (30) and the counterforce system (40) are respectively arranged above and below the confining pressure system (20); and a vertical displacement meter of the automatic data acquisition system (50) is arranged in the sample (10). Compared with the prior art, an original structure of the sample can be maintained; confining pressure can be applied to the sample; an in-situ stress field of a measured soil body can be reproduced; the soil body can be damaged along the weakest surface thereof; and the size of the sample is large, so that the size effect influence is reduced. The large-scale in-situ triaxial shear test device and the method thereof have the characteristics of being real and comprehensive on the issue of researching the shear strength of the soil body; and test data are reliable, and powerful guarantee is provided for design and construction in the field of geotechnical engineering.